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Performances of two pigeons were studied on a concurrent discriminated TO avoidance‐VR schedule. Each avoidance response postponed a TO from a VR 140 for a specified RS interval. The warning stimulus on the TO avoidance schedule was a discontinuous clock which consisted of a series of discrete color changes that varied systematically with the RS interval. Experimental manipulations established that the avoidance behavior was under the control of the avoidance schedule and the discontinuous clock. Five‐min TOs maintained higher avoidance rates than shorter TO durations; a 15‐min TO maintained less avoidance responding than the 5‐min TO. Chlorpromazine hydrochloride increased TO avoidance behavior and decreased concurrent VR behavior.

Pigeons were shown to come under discrimination control when the SD and SΔ were temporally separated from reinforcement and non‐reinforcement. SD and SΔ consisted of distinctive key illuminations presented separately. Responding on an FR 5 in the presence of SD or SΔ produced a third stimulus containing a schedule requirement. If this third (or interpolated) stimulus was preceded by SD, responding in its presence produced reinforcement followed by a time‐out (TO). If, on the other hand, the third stimulus was preceded by SΔ, responding produced TO alone. In this fashion, the same stimulus and the same response requirement were imposed between SD and the reinforcement as between SΔ and the TO. In Experiment I, the schedule employed during the interpolated stimulus was FR; in Experiment II, FI. Discrimination reversal was accomplished in both experiments.

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Pigeons were trained to peck either of two response keys for food reinforcement on equated aperiodic schedules. Delays of reinforcement for pecks at one key reduced the relative frequency of pecking exponentially as a function of the delay interval. Similar functions were obtained when other dependent variables were plotted against the delay interval.

Pigeons were trained to peck for food reinforcement, and the minimum effective force for a peck was varied. In a single‐key situation, the response rate was unaffected up to a certain force value, beyond which increments caused a proportional decrement in response rate. Transient enhancement and suppression effects were observed following a change in the force requirement. A change from a high force requirement to a low force requirement resulted in a temporary enhancement of response rate above the stable performance level, whereas a change from a low to a high requirement produced a temporary suppression of the rate below the stable rate. In a two‐key situation, the response rate on a key with a given force remained unaffected by the force requirement on the other key. When the rate of response was plotted against the required force, the resultant function was remarkably similar to that obtained from the single‐key experiment. When the rates of reinforcement on the two keys differed, the decrement in response rate produced by an increase in the force requirement was proportionally greater on the key with the lower rate of reinforcement.

Squirrel monkeys were exposed to brief tail‐pinches in the presence of a cloth‐covered ball. Attack was elicited against the ball as a direct function of the force of the tail‐pinch. This finding in conjunction with previous findings regarding electric shock and intense heat demonstrates that several types of aversive stimulation can elicit aggression.

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