The role of the hippocampal system in learning and memory processes in the chick was investigated. A series of experiments examined the effects of lesions in the hippocampal system on the acquisition and retention of a passive avoidance task. Chicks given pretraining bilateral hippocampal lesions showed a decrease of retention of the avoidance response evaluated 3 h posttraining. When given unilaterally, left, but not right lesions, resulted in reduced avoidance. However, bilateral posttraining lesions, made 1 h after training, did not interfere with retention of the task. These results suggest an involvement of the hippocampal system in learning processes in the chick.