This study tested for sex differences in the effects of chronic nicotine administration and withdrawal on nicotinic acetylcholine receptor binding in brain. Rats received nicotine (0.6 mg/kg, s.c.) or saline once daily for 15 days, and were sacrificed 1 or 20 days after termination of treatment. Saturation studies of nAChR binding were performed using [3H]cytisine as the radioligand in whole brain minus cerebellum taken from animals in the chronic treatment groups and from naive rats. Male but not female rats that received chronic nicotine had higher receptor densities than corresponding control groups; up-regulation of nAChR was not seen 20 days after withdrawal. Furthermore, in groups that showed no up-regulation (controls and rats withdrawn for 20 days), nAChR densities were higher in female rats than males. The findings underscore the importance of sex differences in pharmacological responses as well as in basal neurochemical parameters.