Intracerebral injection of L-proline, 1 minute after one-trial avoidance training of chicks, impaired memory 24 hours later. Chicks injected with proline 1 or 4 hours after training and controls injected with L-isoleucine at intervals after training, showed no impairment of memory 24 hours after training. The retrograde impairment of long-term memory induced by proline occurred without the convulsions or electrographic events usually associated with retrograde amnesic treatments.