The effect of active immunization against nicotine on the initial distribution of nicotine to brain was studied in anesthetized rats. Animals received nicotine 0.03 mg/kg nicotine (equivalent to the nicotine dose absorbed by a human smoking two cigarettes) as a rapid injection in the jugular vein. In control animals, the arterial serum nicotine concentration initially exceeded the venous concentration 4.6-fold, similar to the initial arteriovenous difference produced by cigarette smoking in humans. Animals immunized with the nicotine analog CMUNic maintained this arteriovenous gradient, but with both arterial and venous nicotine concentrations several times higher than in controls. The arterial nicotine concentration was higher in immunized animals even at the first (7.5 s) sampling time. The brain nicotine concentration at 3 min was 36% lower in the immunized animals. The time course of nicotine distribution to brain was studied in a second group of animals. Brain nicotine concentration was reduced in rats immunized with CMUNic over the entire 6-min sampling period immediately following nicotine dosing (mean reduction 38%). A reduction was found at the earliest sampling time (30 s) and was maximal at 1 min (48%). Nicotine protein binding in serum was markedly increased in animals immunized with CMUNic compared to controls (91.2 versus 10.9%), and the unbound nicotine concentration in serum was lower (10.0 versus 13.4 ng/ml). The reduction in brain nicotine concentration correlated with antibody affinity for nicotine, and the percentage of nicotine bound in serum. These data demonstrate that nicotine-specific antibodies produced by active immunization rapidly bind nicotine in arterial blood, reduce the unbound nicotine concentration, and reduce the early distribution of nicotine to brain. Effects were observed using a clinically relevant nicotine dose and route of administration. These data suggest that the use of immunization to modify the behavioral effects of nicotine may be possible.