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Pigeons were conditioned to peck a response key under a procedure that alternated periods of food reinforcement with periods of extinction. The pigeons attacked a nearby pigeon at the onset of extinction. Some also attacked a stuffed model of a pigeon. The duration of attack was an inverse function of the time since the last food reinforcement and a direct function of the number of reinforcements. The pigeons attacked after the last food delivery whether or not the conditioned pecking response was required and whether or not the extinction period was signaled. The food had to be eaten; the mere sight and sound of food being delivered did not produce attack. Prior satiation reduced attack. The phenomenon was not attributable to a past history of competition between pigeons since socially deprived pigeons also attacked. Superstitious reinforcement of attack was not found to be a factor. The results indicated that the transition from food reinforcement to extinction was an aversive event that produced aggression.

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Five 14½‐ to 19½‐month‐old infants were trained to lever press for snacks on small fixed ratio schedules of reinforcement. Within four to nine sessions, responding under FR 10 was established for four subjects and FR 15 for the other. Each subject's last session revealed behavioral patterns similar to animal and human FR trained subjects—a high and constant ratio rate, mixed with a zero rate following reinforcements. Deviations were mostly in the form of prolonged and variable post‐reinforcement pauses. These and other irregularities were probably due to the limited deprivation conditions and improper training procedures in which the ratio (for two subjects) was ascended too early and too quickly. Extinction was instituted during the last session. The degree to which extinction performance matched that of other organisms depended upon how stable and “ratio‐like” performance was during conditioning.

Critical flicker fusion frequencies were determined as a function of stimulus intensity for three White Carneaux pigeons using a modified conditioned suppression paradigm as a threshold procedure. Critical frequencies ranged from 6 cps for the lowest intensity of 0.0014 ml to 77 cps for the maximum intensity of 41.86 ml.

Two temporal parameters of free operant or Sidman avoidance behavior are the interval by which responses postpone shocks (Response‐Shock interval) and the interval between shocks when no responses occur (Shock‐Shock interval). Avoidance behavior was examined in three white rats under conditions where the Response‐Shock and Shock‐Shock intervals were always equal. With intervals from 10 to 60 sec response rates and shock rates were similar, decreasing, negatively accelerated functions of increasing Response‐Shock = Shock‐Shock interval. Over this range, response and shock rates were linearly related to the reciprocal of the Response‐Shock = Shock‐Shock interval. It was shown, however, that this relation cannot hold at extremely long intervals. Both the ratio of responses emitted to shocks received and the percentage of shocks possible which were avoided increased at long Response‐Shock = Shock‐Shock intervals. These findings may be related to the fact that long intervals provide optimal conditions for conditioning avoidance behavior in the rat.