Previous studies indicate that the Ginkgo biloba extract, EGb 761, has a facilitative effect on deficient memory. The temporal parameters of this effect, however, have not been clearly defined or distinguished from the effect on normal memory. The aim in the current study was to investigate the effects of EGb 761 on memory using a well-controlled animal model. Day-old chicks were trained on either a weakly or strongly reinforced version of a passive avoidance task. Long-term memory formation of the weakly reinforced version of the task was improved significantly by EGb 761 (3 mg/ml) when administered between 10 and 30 min after training. However, the same dose of EGb 761 impaired retention when administered prior to strongly reinforced training. These data provide convincing evidence that posttraining administration of EGb 761 initiates long-term memory in chicks with only short-term memory, but that the same dose-administered pretraining can be deleterious for normal retention. This dual effect has important implications for the clinical use of Ginkgo biloba extracts.