Following the observation of increased trough whole blood cyclosporin A (CyA) concentrations and reduced renal function in a patient with recalcitrant generalized pustular psoriasis treated with a combination of CyA and etretinate, the effect of vitamin A analogues on human microsomal cytochrome P450-dependent CyA metabolism was investigated in vitro. In addition, the effect of terbinafine, a new allylamine antifungal agent, was also tested. Etretinate, its major metabolite acitretin, and isotretinoin, each at a single concentration of 100 microM, inhibited total hepatic microsomal CyA metabolism to a similar extent (33-45%, compared with control values). The generation of total primary and total secondary CyA metabolites was also inhibited to a similar extent by each of the retinoids. Conversely, terbinafine was without significant effect on CyA metabolism in vitro. The results, which suggest that inhibition of hepatic CyA metabolism by retinoids may contribute to increased circulating CyA concentrations, are discussed in relation to other potential drug interactions, and to the use of etretinate in reducing the CyA administered dose.