The high selectivity of amphetamine and its derivatives for CYP2D-mediated oxidations suggested the use of the phenylisopropylamine skeleton as a template for a selective inhibitor of this important enzyme. Accordingly, 4-allyloxymethamphetamine-amine (ALLMA) was synthesized and its ability to selectively inactivate CYP2D was investigated both in in vitro and in vivo experiments. Incubation studies with rat liver microsomes demonstrated that this compound suppressed the CYP2D-mediated methylenedioxymethamphetamine (MDMA) demethylation in time- and dose-dependent manner and that the inhibition required the presence of NADPH. The development of irreversible inhibition was associated with oxidation at position 4 of the aromatic ring, the common site of CYP2D-mediated oxidation of this group of compounds. In in vivo studies doses of ALLMA (1-10 mg/kg) were administered to adult male Sprague-Dawley rats and liver microsomes were obtained 3 hr later. Methamphetamine p-hydroxylation and low Km MDMA demethylation activities, both mediated by CYP2D, were reduced by more than 80% after a dose of 10 mg/kg. Cytochrome P-450 reactions attributed to P-450s other than CYP2D, such as aniline p-hydroxylation, the high Km system of MDMA demethylation and the N-demethylation of methamphetamine, benzphetamine, aminopyrine and erythromycin, all appeared to be minimally affected. The importance of aromatic ring oxidation in the metabolism is such that inhibition of CYP2D would be expected to cause a significant change in the pharmacokinetics of these compounds. The kinetics of MDMA metabolic activity in microsomes from ALLMA-pretreated rats were comparable to those from female Dark-Agouti rats, an animal model for CYP2D1 deficiency.