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Goldfish trained in a shuttle box under conditions in which changing compartments postponed shock for 20 sec showed a substantial, negatively accelerated increase in rate of crossing. That the avoidance‐contingency was responsible for the change in behavior is suggested by the fact that no significant increase in rate of crossing appeared in control animals which were paired with the experimentals and shocked whenever the experimental animals were shocked; there is some evidence, indeed, that the control animals were handicapped in their subsequent adjustment to the avoidance condition. The introduction of a warning stimulus (light) in the last 5 sec of the response‐shock interval decreased the rate of crossing in the first 15 sec and increased the rate of crossing in the last 5 sec. Reducing the shock‐shock interval from 20 sec to 2.5 sec had no marked effect. The results are compared with those obtained in analogous experiments with higher animals.

Auditory frequency discrimination thresholds have been determined by a “tracking procedure” at five frequencies and two intensity levels for a group of four cats. Intra‐animal thresholds were found to be quite stable, and there was good inter‐animal agreement among three of the four cats. Although quite stable, the tracking thresholds were larger than those which had been obtained using an avoidance procedure. However, because of the several differences between the tracking procedure and avoidance conditioning procedure, no conclusions can be drawn concerning the efficacy of the different types of reinforcement in determining the ultimate limit of the animals' discrimination ability.

When the individual SD components of a multiple schedule were combined, their control over a response summated, thus increasing the response probability to a point over that controlled by either of the SDs independently. Summation was concluded to be a phenomenon relevant for operant as well as respondent stimulus control (Pavlov, in Kimble, 1960; Hull, 1943). The results of the present study appear to be a special case of the general SD enhancement effect demonstrated by Hanson (1959) and by Pierrel and Sherman (1960).

A pigeon's rate of pecking on a red key, reinforced at a constant frequency, may be changed by increasing or decreasing the frequency of reinforcement of pecking on a successively presented green key. The changes in the rate of pecking on red, called interactions, are of two types: contrast, in which the changes in the rates of pecking on the two colors are in opposite directions; and, induction, in which the changes in the rates are in the same direction. In previous data, a change in the frequency of reinforcement associated with the green key produced a corresponding change in the rate of pecking the green key and an opposite change (contrast) in the rate of pecking on the red key. The present data suggest that the magnitude of contrast is very small if pecking on the red key is reinforced at a high enough frequency (about 40 reinforcements per hr in the present experiment). Also, given that interactions occur, induction rather than contrast may result from small changes in a low frequency of reinforcement associated with green.

Six adult female albino rats were subjected to operations known to increase the frequency of wheel‐running. Operations included limited access to wheel, food deprivation, and protracted maintenance on a 24‐hr feeding schedule. A distributional analysis of response duration, burst duration, and inter‐burst interval showed that the increased frequency in all cases arose mainly from a shortening of the interval between successive bursts. Only short‐term food deprivation produced any notable increase in burst length, and even here changes in both speed of the individual responses and in number of responses per burst were slight compared to changes in inter‐burst interval.

Three years ago a tone ending in unavoidable electrical shock was periodically presented to pigeons while they pecked a key for food. When pecking was disrupted by tone, shock was disconnected and the training tone as well as tones of different frequencies were presented. At first, all tones caused a reduction in the rate of pecking, but as testing proceeded, suppression began to extinguish and the gradient narrowed. In the present work, testing was resumed after a 2½‐yr interruption. Analysis of the gradients obtained just before and just after the interruption yielded no evidence of changes with the passage of time. As testing proceeded, however, extinction of suppression continued and the gradient all but disappeared. In subsequent experiments with these subjects (Ss) it was found that the presentation of free shocks caused a reappearance of the gradient and that this effect persisted in reduced amount for several sessions after the shock condition was terminated.

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