Chicks, age 1 to 2 days, that have been trained on a passive avoidance task with a strongly reinforced training trial yield a memory trace that is composed of 3 behaviorally and pharmacologically distinguishable stages, with the final long-term memory stage being dependent on protein synthesis. In contrast, chicks trained with a weakly reinforced learning trial typically do not demonstrate this final stage of memory. Sodium nitroprusside 150 microM intracranially administered immediately after a weak training trial promoted the formation of long-term memory, whereas saline did not. The results suggest that nitric oxide synthesis is either itself critical or stimulates other processes that are critical for the consolidation of long-term memory.