To assess the effects of pargyline (555577) on rat hepatic microsomal parameters and its metabolism by enzymes present in microsomes, Sprague-Dawley-rats were injected daily intraperitoneally with pargyline for 3, 7, or 14 days, at doses of 75mg/kg or 15mg/kg. Doses of pargyline at 75mg/kg significantly reduced ethylmorphine-N-demethylase activity and cytochrome-P-450 content. Doses of pargyline at 15mg/kg did not alter ethylmorphine-N- demethylase activity or cytochrome-P-450 content. In-vitro studies revealed that pargyline was metabolized to at least three compounds by rat hepatic microsomes. Thin layer chromatographic data suggest that one metabolite was an N-demethylated form, norpargyline.