Low doses (0.12-2.0 μg/kg) of cholecystokinin octapeptide (CCK), administered intraperitoneally, support the formation of conditioned odor preference in neonatal and weanling rats. Exposure to a novel odor was paired with CCK injection, and the rats' olfactory choices were assessed 24 hr later. Rats at 5, 11, and 22 days of age preferred the odor previously associated with CCK, compared with vehicle-injected littermates. In contrast, CCK failed to support olfactory conditioning in 28-day-old rats, whether they were (a) weaned and independently housed, (b) residing with the dam and suckling, or (c) fed only milk. Adult rats also did not establish an odor preference with CCK as the unconditioned stimulus. Thus, CCK's changing impact from positive to neutral probably occurs during the rats' 4th postnatal week and may be related to maturational changes occurring during the final stages of weaning.