Glucocorticoids are released during learning situations and can trigger neural actions through binding to receptors in different brain areas. The possible role of a glucocorticoid action in long-term memory formation was studied, in day-old chicks, by using a passive avoidance task which chicks otherwise only retain for a few hours (< 10) after training. Thus, we examined the effects of intracerebral corticosterone administration on retention 24 h posttraining. The results showed that chicks injected with corticosterone (1 microgram) at either 15 min pretraining or at 5, 30, 60 min (but not 120, 180, or 360 min) posttraining retained the passive avoidance response when tested 24 h posttraining. Studies with specific mineralocorticoid or glucocorticoid receptor antagonists (RU 28318 or RU 38486, respectively) indicated that this increase in retention by corticosterone might be mediated through glucocorticoid receptors. In order to assess whether the facilitatory effect of corticosterone was mediated through an effect on protein synthesis mechanisms, the protein synthesis inhibitor anisomycin was administered prior to corticosterone. However, this treatment only partially attenuated the effect of the steroid, suggesting that corticosterone may influence other cellular processes involved in the formation of long-term memory for the avoidance behaviour.