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Six pigeons pecked for food in a three‐key experiment. A subject at any time could choose the left or right key and receive reinforcement according to one two‐key concurrent variable‐interval variable‐interval schedule of reinforcement, or it could peck the center key. A peck on the center key arranged the complementary two‐key concurrent variable‐interval variable‐interval schedule on the left and right keys. The two different two‐key concurrent schedules arranged reinforcements concurrently and were signalled by two different colors of key lights. Choice behavior in the presence of a given color conformed to the usual relationship in two‐key concurrent schedules: the relative frequency of responding on a key approximately equalled the relative frequency of reinforcement on that key. Preference for a two‐key concurrent schedule, which was equivalent to preference for a color, was measured by the percentage of all responses on the left and right keys in the presence of that color: this percentage approximately equalled the percentage of all reinforcements that were delivered in the presence of that color. Thus, choice between concurrent schedules conforms approximately to the same relationship as does choice between alternatives in a single concurrent schedule.

Rats chose between two signalled avoidance schedules. Under one schedule, responses in the presence of the signal terminated it and resulted in avoidance of shock; in the other, responses in the presence of the signal resulted in shock avoidance but signal termination was delayed for 10 sec. Pressing a second (changeover) lever produced change from one schedule to the other for 1 min. Once this 1‐min period timed out, subjects could remain under the schedule in effect or could reinstate the other schedule for another 1‐min period. All four subjects continuously changed over from the non‐terminating to the terminating signal schedule. Changeover responding was not maintained when the termination contingency was removed. When changeover responding resulted in a change from a terminating to a non‐terminating signal schedule, changeover responding did not occur.

Unsignalled, inescapable shocks were presented to four albino rats in Experiment 1. By pressing a lever subjects could change the condition to signalled shock for 3‐min periods after which unsignalled shock was automatically reinstated. All subjects changed from unsignalled to signalled shock when shock density was the same or when the density of signalled shock was two times greater than unsignalled shock. When the density of signalled shock was four times that of unsignalled shock, three subjects changed to the higher density schedule. One subject changed to a density of signalled shock eight times that of unsignalled shock. The second study showed that the two shock schedules most similar in Experiment 1 were discriminably different because subjects chose lower over higher shock densities when both densities were unsignalled. An analysis stressing safe (signal absent) and unsafe (signal present) periods was discussed.

Unsignalled, inescapable shocks were presented to four albino rats in one study and to six rats in a second study. By pressing a lever, subjects could change the condition to signalled shock for 3 min after which unsignalled shock was automatically reinstated. All subjects changed frequently to the signalled shock schedule. After a minimum of three 6‐hr sessions or after changeover responding stabilized at the previous values, higher values of signalled shock intensity or duration were introduced. In the first study, the duration of signalled shock was increased in increments of 0.5 sec. In the second study, the intensity of signalled shock was increased in increments of either 0.2 or 0.4 mA. Duration subjects chose signalled shock four (2.0 sec) to nine times (4.5 sec) longer than unsignalled shock (0.5 sec). Intensity subjects chose signalled shock two (2.0 mA) to three times (3.0 mA) more intense than unsignalled shock (1.0 mA).

An observational technique for reliably estimating the per cent of time a student engages in appropriate and inappropriate classroom behavior is described. The regular classroom teacher can utilize the procedure without deviating from regular routine, and the obtained data can serve as a basis for dispensing token reinforcement.

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Responding maintained in rats under a variable‐interval 35‐sec food schedule was suppressed more by 60‐sec and 240‐sec fixed‐interval schedules of shock delivery than by 60‐sec and 240‐sec variable‐interval schedules of shock delivery. When the delivery of shock was preceded by a 5‐sec visual stimulus, little overall response suppression was found with either fixed‐interval or variable‐interval schedules. In a third experiment, the percentage of occasions on which a cue preceded each shock delivery was varied from 0% to 100%. For the fixed‐interval shock condition, the most suppression occurred with the 0% treatment, the least with 100%, and an intermediate amount with the 50% treatment. For the variable‐interval groups, the most suppression occurred in the 50% condition, the least in the 100% group, and an intermediate amount with 0%.

In Experiment 1, rats' responses were reinforced on a fixed‐interval 30‐sec schedule in the presence of either a light or a tone and were not reinforced in their absence. Each stimulus was correlated with its own response lever, with only one lever present during a session. When light and tone were compounded in the presence of the tone‐correlated lever, no change in responding occurred. However, when tone was compounded with light in the presence of the light‐correlated lever, level of responding was greater than to light alone (response summation). Summation was also found when each stimulus was correlated with the same lever. Next, light and tone were again correlated with separate levers, but both levers were always simultaneously present. Compounding produced both summation and emission of most responses on the light‐correlated lever. This prepotency of light was reduced (1) by leaving a houselight on throughout the session; and (2) by correlating each stimulus with a different schedule (either fixed‐interval 4.7‐sec or fixed‐interval 30‐sec). With a medium‐ and high‐intensity houselight and with the different reinforcement schedules, similar results were obtained during compounding, regardless of whether compounding occurred in the presence of the light‐ or tone‐correlated lever.