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The effects of differential reinforcement of vocal duration were examined in a series of experiments in which each of 28 subjects (Ss) emitted a vowel whenever a light was flashed. In the first phase of each experiment, a penny was dispensed after each of 20 responses. In the second and subsequent phases, only those responses whose durations exceeded a criterion were reinforced; when 10 successive reinforcements were presented, one phase was terminated and the next begun. The criterion for reinforcement in each phase was determined by a different schedule in each of six experiments; it ranged from 80 to 120 per cent of the mean duration of the 10 terminal responses in the prior phase. Differential reinforcement effected a large and systematic change in the duration of vocal responses as long as the responses selected for reinforcement had a sufficiently high probability of occurrence. This requirement was formulated as the difference between the criterion duration and the mean duration of the terminal responses in the prior phase, divided by their standard deviation. This statistic, named the shaping index, was correlated with the number of responses emitted before each phase was terminated. It was found to be large whenever the shaping process failed. Many Ss failed to tact the reinforcement contingency despite marked changes in their vocal behavior and extensive probing by a questionnaire, administered at the end of each session.

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The effect of CER on DRL 16 was studied in four rats. All Ss showed complete CER suppression after five CER trials, together with some unconditioned post‐shock suppression. This post‐shock suppression showed complete recovery in all Ss after 10 days of five CER trials per day, but complete CER suppression continued throughout.

The effect of Pavlovian discrimination training with two stimuli upon subsequent learning of an operant discrimination involving those stimuli was studied. After preliminary lever press training, the lever was removed and thirsty rats received noncontingent pairings between S1 (a tone or a clicker) and water reinforcements, whereas S2 (a clicker or a tone) occurred always without reinforcement. This procedure presumably established S1 as a positive CS for respondent behavior, whereas S2 was established as an inhibitory CS. Following this training, the lever was reintroduced and the rats were trained on an operant (lever pressing) discrimination involving S1 and S2. For the Consistent Ss, S1 was the SD and S2 the SΔ in the operant discrimination; for the Reversed Ss, S2 served as SD and S1 as SΔ. The Consistent Ss learned the operant discrimination significantly faster than did the Reversed Ss. The result emphasizes the importance of respondents, conditioned to SD and SΔ, which modulate operant performance to these stimuli.

Shock‐induced fighting between paired rats was a direct function of the duration of the shock stimulus. Continued presentations of the shock partially reversed this direct relationship.

Following 30 days of reinforcement for the bar press response of two white rats on 30‐sec fixed‐interval (FI), a DRL component was added so that a minimal interresponse time (IRT) for the reinforced response, in addition to the FI variable, was necessary for reinforcement. Marked control over response rate by the superimposed DRL requirement was demonstrated by an inverse hyperbolic function as the DRL component was increased from 1 to 24 sec within the constant 30‐sec FI interval. Interresponse time and post‐reinforcement (post‐SR) “break” distributions taken at one experimental point (DRL = 24 sec) suggested that a more precise temporal discrimination was initiated by an SR than by a response, since the relative frequency of a sequence of two reinforced responses appeared greater than that of a sequence of a non‐reinforced response followed by a reinforced one. This latter finding was confirmed with new animals in a follow‐up experiment employing a conventional 24‐sec DRL schedule.

Rats were trained to press two keys consecutively for reinforcement. During stimulus one (slow clicker) a 6‐sec time delay was required between the two responses. During stimulus eight (fast clicker) no time delay was required between the two responses. When tested with intermediate stimuli (intermediate click rates) the median time delays emitted by the animals were intermediate between their performances on the original training stimuli, resulting in typical generalization gradients. Closer examination of the data revealed that the median values were not representative of the behavior of the animals.

Inter‐response time distributions with class intervals of 0.05 and 0.04 sec revealed characteristics not observable with class intervals of 0.5 sec or greater. Bimodalities were clearly evident in the inter‐response time distributions for two out of three pigeons. These modes seemed to correspond to two discrete response topographies.