This study examines the effects of ketoconazole, R 75 251 and some other cytochrome P-450 inhibitors on the in vivo metabolism of all-trans-retinoic acid (RA) in normal rats. Oral treatment with ketoconazole or R 75 251 (40 mg/kg, -1 hr) reduced the elimination rate of i.v. injected RA from plasma: the half-life of RA increased from 27 min in control-treated animals to 43 min and 76 min after dosing with ketoconazole and R 75 251, respectively. However, neither drug had an effect on the distribution volume of the retinoid. Two hours after i.v. injection of RA, residual plasma levels of the retinoid were 11.2 ng/ml in ketoconazole and 22.7 ng/ml in R 75 251-treated rats. The other P-450 inhibitors, aminoglutethimide, cimetidine, itraconazole, metyrapone and saperconazole, showed no sparing effect on RA elimination: plasma levels of the acid were below 1 ng/ml, as in control-treated animals. Administration of ketoconazole or R 75 251 (40 mg/kg, -2 hr) to rats also enhanced endogenous plasma concentrations of RA. Levels of the retinoid were raised from mostly undetectable values (less than 0.5 ng/ml) to 1.3 +/- 0.1 and 2.5 0.1 ng/ml after treatment with ketoconazole and R 75 251, respectively. These data are indicative of the important contribution of the cytochrome P-450 enzyme system to the in vivo metabolic process of RA. In vivo inhibition of the P-450 pathway not only increased the biological half-life of exogenously administered RA, but also enhanced the endogenous plasma level of this vitamin A derivative.