This study investigates the functional role of corticosterone in memory formation for a passive avoidance task in the day-old chick. Whereas training chicks with a strong aversant results in an enduring memory, memory for a weak aversant which is only retained for a few hours (< 9 h) is facilitated by intracerebral corticosterone administration. We now report that only chicks trained on the strong task, a learning situation that results in a high percentage of chicks (around 80%) forming a long-term memory, showed increased post-training plasma corticosterone levels, whereas chicks trained on the weak task showed corticosterone values comparable to untrained chicks. We also questioned whether the effects of corticosterone on retention might be concentration dependent. In the weak task, intracerebral administration of 1 microgram corticosterone facilitated retention, but a higher dose failed to induce this effect. However, in the strong task, corticosterone administration at doses of 1 and 5 micrograms produced an impairment in long-term retention for the avoidance response. These results support a crucial role of corticosterone release following training on the physiological mechanisms ensuring the transition from short- to long-term memory, and provide an animal learning model for the study of the mechanisms of action of a biphasic modulation of memory formation by acute corticosterone administration.