The site of action of corticosteroids in avoidance learning was investigated in 110 rats. Injection of cycloheximide, 30 min before one-trial training on a passive avoidance task suppressed corticosteroid secretion in response to footshock, and produced an avoidance deficit in a test 6 days later. However, an additional injection of hydrocortisone, either subcutaneously or intra-hippocampally within 5 min of training, restored the avoidance response in the test. Septal and hypothalamic injections of the hormone were ineffective in reversing the cycloheximide effect, whereas the effect of hormone injection into the amygdala was equivocal because of an increased level of activity. Corticosteroids secreted following an aversive experience appear to act upon the steroid-sensitive neurons in the hippocampus to influence the animal's later performance of passive avoidance response.