In mammals, P-glycoprotein immunostaining at the blood–brain barrier has implicated the multidrug pump in the restricted movement of many cytotoxic agents into the central nervous system (NCS). Since many insects require as sophisticated blood–brain barrier system to protect their CNS from plant-derived neurotoxins, we have investigated the possibility that a P-glycoprotein homolog constitutes a component of the insect blood–brain barrier. We have used the nicotine-resistant tobacco hornworm (Manduca sexta) to address this issue. Manduca has been previously shown, in physiological studies, to have an alkaloid (nicotine/morphine/atropine) pump at its excretory malpighian tubules. We show (1) that the tubules are P-glycoprotein immunopositive, (2) that Manduca has a metabolic blood–brain barrier for nicotine, (3) that the barrier co-localizes with P-glycoprotein immunostaining, and (4) that detoxifying enzymes as well as the nicotine pump are likely to account for the metabolic blood–brain to nicotine. These findings may provide insights on two major fronts, the troublesome problem of multi-insecticide resistance, a phenomenon that parallels multidrug resistance in tumor cells, and the problem of tolerance to addictive neuroactive drugs like nicotine or morphine. 1994 John Wiley & Sons, Inc.