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A variety of methods, definitions, and theoretical notions that have been used in the study of inhibitory stimulus control were reviewed and evaluated. Preliminary data from several new operant methods were also described. It was proposed that future workers distinguish clearly between two forms of inhibitory control: (a) the learned power of a specific stimulus to reduce behavior, and (b) a dimensional effect, in which responding increases as values progressively more distant from the value of that specific stimulus along some dimension are presented (generalization gradient). Conclusions from several important recent studies were shown to be strongly dependent on the individual experimenter's criterion for deciding when a stimulus is inhibitory. The concept of inhibition seems a very valuable one for the field of operant behavior, and it deserves more attention than it has received in the past.

Six pigeons were exposed to two keys, a main key and a changeover key. Pecking the main key was reinforced on a variable‐interval 5‐min schedule when the key was blue and never reinforced when the key displayed a vertical line on a blue background. Each peck on the changeover key changed the stimulus displayed on the main key. Each subject was given two generalization tests, consisting of presentations on the main key of six orientations of the line on the blue background, with no reinforcements being given. In one test changeover‐key pecks changed the stimulus; in the other test the changeover key was covered and the experimenter controlled stimulus changes. Both responses to the six stimuli and time spent in the presence of the stimuli gave U‐shaped gradients when the changeover key was operative. With most subjects, absolute rates of responding to each stimulus produced unsystematic gradients, whether or not the changeover key was operative.

When given pre‐conditioning instructions correctly indicating the maximum number of reinforcements obtainable, subjects made few responses during extinction following FR 10 conditioning. More extinction responses occurred when the maximum‐reinforcement instructions suggested that reinforcements were obtainable during extinction. The highest rates of responding during extinction were produced by subjects who had no maximum‐reinforcement instructions.

The times between each of the first thirteen responses after reinforcement (the first twelve interresponse times) were determined for two pigeons whose pecking was reinforced on fixed‐interval schedules of food reinforcement ranging from 0.5 min to 5 min. These interresponse times were classified with respect to their ordinal position in the sequence of responses and with respect to the time since the preceding reinforcement at which the initiating response occurred. The median interresponse time durations were essentially constant after the sixth response after reinforcement regardless of the time at which the interresponse time was initiated. The durations of the first few interresponse times after reinforcement decreased as the number of preceding responses increased and as the time since the preceding reinforcement increased.

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