Microsomal P450 monooxygenases contribute actively to the biotransformation of the antiglucocorticoid RU38486, an 11 beta-substituted nor-steroid. Pretreatment of adult rats by inducers of specific forms, belonging to different P450 subfamilies, affects the ability of liver microsomes to metabolize RU38486. Phenobarbital and pregnenolone 16 alpha-carbonitrile increase the metabolic activity of liver microsomes whereas methylcholanthrene decreases their capacity to oxidize the steroid. Thus P450 forms IIIA, IIB1,2 and IIC7 are good candidates to be involved in the degradation of this peculiar molecule. Our study has been completed by investigating whether RU38486 would influence the P450 spectrum. Whereas the treatment of rats with either a glucocorticoid (cortisol, dexamethasone) or an antiglucocorticoid (pregnenolone 16 alpha-carbonitrile) has been shown to induce the P450 activity by increasing the hepatic concentration of form IIIA, we observed a slight decrease of the P450 activity by treating the animals with RU38486. Moreover RU38486 was able to antagonize the P450 induction by the other steroids as well as it inhibits the synthesis of various liver enzymes induced by glucocorticoids (for instance tyrosine aminotransferase). These findings may be important for the therapeutic use of RU38486 since its inhibitory effect on P450 activity may be at the origin of drug interactions by modifying the endogenous hormonal status.