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Pigeons were exposed to a schedule of stimulus‐correlated food presentations. When key pecks terminated trial signals and cancelled the delivery of food, pecking was either gradually or rapidly redirected away from the keys, depending on whether the food‐omission contingency was introduced from the outset or after exposure to a response‐independent baseline. In all cases, the food‐omission contingency substantially reduced or eliminated pecking at the keys.

Pigeons' pecks at two white response keys (initial‐link situation) occasionally turned both keys red (terminal‐link situation). When the two keys were red, pecks occasionally produced food, after which the keys were again white. In both situations, a changeover delay prevented the response‐produced outcome from immediately following a change of responding from either key to the other. In the initial‐link situation, the ratio of pecks at the keys closely paralleled the ratio of transitions into the terminal‐link situation produced by the pecks, conforming to the well‐known matching relation. In the terminal‐link situation, the peck ratios deviated from the matching relation toward indifference. Overall response rate and rate of changeover were generally higher in the terminal‐link situation than in the initial‐link situation. The finding of matching in the initial‐link situation supports a definition of reinforcement as situation transition. The differences in performance between the two situations, viewed in the light of other recent findings, suggest that the effects of a changeover delay depend on the overall reinforcing value of the choice alternatives.

Rats performed on progressive‐ratio schedules that required an increasing number of responses for each successive reinforcement. The number of responses required increased until the subjects failed to complete the next ratio in the sequence within 15 min. Response‐ratio increments of two responses, five responses, and 20 responses were investigated. The size of the final completed ratio generally increased with increases in the progressive‐ratio step size. Increased pressures of air in a hyperbaric chamber led to both increases and decreases in terminal ratio size, with the differential effects depending on both air pressure and on the size of the progressive‐ratio increment. Changes in the number of responses in the final ratio were related to increased pressures of nitrogen, as similar pressures of helium produced few effects.

Three pairs of pigeons were trained to peck at two keys presented simultaneoulsy in discrete trials with intertrial intervals of 1, 22, or 120 sec. Left‐key responses incremented the probability of reinforcement for the first right‐key response and, conversely, right‐key responses incremented the probability of reinforcement for the first left‐key response. In terms of relative response rates, it was found that all birds' choices were described by a momentary maximizing strategy, but this fact was not reflected in the detailed sequential statistics for birds with the longer (22 or 120 sec) intertrial intervals. It was hypothesized that choice behavior, in general, may be accurately described by a momentary maximizing sequence, but that prior failures to demonstrate this were due to “errors” in executing the momentary maximizing sequence. These misappropriated responses, which are hypothesized to be randomly distributed among the responses defining the momentary maximizing sequence, caused successive choices to appear to be statistically independent when, in fact, they were not.

Three pigeons were required to peck a single key at a higher and a lower rate, corresponding to two classes of shorter and longer concurrently reinforced interresponse times. Food reinforcers arranged by a single variable‐interval schedule were randomly allocated to the two reinforced interresponse times. The absolute durations of reinforced interresponse times were varied while the total reinforcements per hour was held constant and the relative duration, i.e., the relative reciprocal, of the shorter reinforcer class was held constant at 0.70. Preference for the higher rate of responding, as measured by the relative frequency of responses terminating interresponse times in the shorter reinforced class, depended on the absolute reinforced response rates. Preference for the higher reinforced rate increased from a level of near‐indifference (0.50) at high reinforced response rates, through the matching level (0.70) at intermediate reinforced response rates, to a virtually exclusive preference (>0.90) at low reinforced response rates. These results resemble corresponding preference functions obtained with two‐key concurrent‐chains schedules and thereby provide another sense in which it may be said that interresponse‐time distributions from interval schedules estimate preference functions for the component response rates corresponding to different classes of reinforced interresponse times.