The effects of intracerebroventricular administration of two different cholecystokinin antisera were tested on the latency of passive avoidance behaviour of rats following 6, 12, 24, 48, 72 and 96 h retention. In a 1:2 dilution, both CCK antisera caused a shorter latency than in the controls following 6, 12, 24 and 48 h retentions. These data suggest that the endogenous CCK of the brain might be a physiologic modulator of memory processes.