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Rats were reinforced with water for every bar‐press and concurrently punished for every 10th or 20th bar‐press. Punishment produced an initial suppression of responding followed by recovery. A slight change in the method of delivering punishment eventually led to a high response rate just after punishment, a low response rate just before punishment, and frequent intermediate pauses. The results are interpreted as showing that punishment became a safe signal and that the high rate of responding it released came to act as a conditioned aversive stimulus. The effects of amphetamine were consistent with this interpretation. Alcohol had the paradoxical effect of increasing pauses and depressing the low rate before punishment.

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In a discrete‐trials, two‐key choice situation, probability‐learning by pigeons was studied under a variety of training conditions. Matching was found in simultaneous and in successive problems, but a spatial problem produced only maximizing. In the simultaneous problem, noncorrection produced maximizing, while correction produced matching. Guidance produced maximizing when the animals were required to earn each opportunity for choice by pecking a center key on FR‐5, but matching when the center key was not used. In a discrete‐trials one‐key situation, with latency as the measure, frequency and probability of reinforcement were varied independently. Differences in probability produced differences in latency of response, but differences in frequency did not.

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Stimulus generalization was investigated using institutionalized human retardates as subjects. A baseline was established in which two values along the stimulus dimension of auditory frequency differentially controlled responding on two bars. The insertion of the test probes disrupted the control established to the two SDs during training. The discrimination was recovered between each test probe and the resulting gradients were stable across 10 test sessions. These gradients, supported by other two‐response generalization studies, indicate that this type of two‐response discrimination training divides the stimulus dimension into two functional classes separated by a region of transition from one class to the other. Each stimulus value in a class, which extends from an SD outward to the functional limit of the dimension, controls a similar proportion of the two responses as each other value in the class. All values on the stimulus dimension control identical response rates with an absence of the usual generalization decrement. The latency of the initial response, however, shows a bimodal gradient with the modes at the SD values.

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