Corticosterone was administered intraperitoneally to rats, and the consequent changes in plasma corticosterone concentration traced. Stress (sham bilateral adrenalectomy) was applied at various times when the plasma corticosterone concentration was changing at different rates, and it was shown that there was a critical rate of rise of plasma corticosterone concentration of about 1\m=.\3 \ g=m\ g/ 100 ml plasma/min in excess of which the stress response was inhibited. Rates of rise less than this did not affect the stress response. A state of ' saturation' of the control system was also demonstrated to result from high plasma corticosterone concentrations when the response to stress was not inhibited as above. A model of the system is suggested to explain these and other data.

Acquisition of discrete-trial lever-press avoidance learning was studied in three experiments. Experiment I compared a new training procedure, which produces rates of leverpress avoidance learning comparable to those obtained in shuttle boxes, with a "conventional", less efficient training procedure. A factorial design was used to compare continuous versus intermittent shock and a long-variable versus a short-fixed signal-shock interval. Learning was best in the groups trained with the long and variable interval and poorest in those trained with the short and fixed interval. Type of shock had no effect. Experiment II separated the effects of duration from those of variability of the signal-shock interval. Fixed and variable intervals of 10 and 60 sec were tested and duration was the only significant factor. Experiment III addressed the effect of the differential opportunity to avoid provided by long signal-shock intervals by varying this interval from 10 to 60 sec in 10-sec steps.Only the 10-sec group showed slow acquisition relative to the others. Analysis of avoidance response latencies showed that the distributions for all groups were positively skewed and that skewness increased with increasing duration of the signal-shock interval. At intervals longer than 20 sec, the animals made progressively less use of their increased opportunity to respond. The data do not support the opportunity-to-respond interpretation of the effects of duration of signal-shock interval and suggest that some type of inhibitory process may block lever-press avoidance learning at intervals as short as 10 sec. The significance of these findings for species-specific defense reaction and preparedness theories was emphasized.

The effect of intertrial and intersession interval on the relearning of an incompletely learned avoidance response was studied in a factorial experiment having three intertrial intervals—½, 1, and 2 min.—and five intersession intervals—.08, 1, 4, 24, and 168 hr. Original learning was avoidance training to a criterion, and relearning was 40 trials of the same training. Five groups of 10 rats, matched for rate of original learning, were trained at each intertrial interval. The number of avoidance responses during relearning was a U-shaped function of intersession interval for all intertrial intervals. During the first 10 relearning trials, the maximum interfering effect occurred after an intersession interval of 24 hr. in the ½-min. groups and after 4 hr. in the 1- and 2-min. groups. During the rest of relearning, maximum interference was reached after 1 hr., and the intersession interval functions were independent of intertrial interval, except for over-all level. These results were interpreted in terms of a “parasympathetic over-reaction” following fear conditioning.

Long-Evans rats were selectively bred for over 20 generations using two behavioral criteria: (1) limited responding to the warning signal during ten pretest trials and (2) either good or poor avoidance behavior during 60 avoidance training trials in a two-way shuttlebox. The results suggest that avoidance behavior in the rat is a heritable characteristic that may be selected independently of activity level.

No abstract