Repaglinide is an antidiabetic drug metabolised by cytochrome P450 (CYP) 2C8 and CYP3A4 enzymes. To clarify the mechanisms of observed repaglinide drug interactions, we determined the contribution of the two enzymes to repaglinide metabolism at different substrate concentrations, and examined the effect of fibrates and rifampicin on CYP2C8, CYP3A4 and repaglinide metabolism in vitro. We studied repaglinide metabolism using pooled human liver microsomes, recombinant CYP2C8 and recombinant CYP3A4 enzymes. The effect of quercetin and itraconazole on repaglinide metabolism, and of gemfibrozil, bezafibrate, fenofibrate and rifampicin on CYP2C8 (paclitaxel 6alpha-hydroxylation) and CYP3A4 (midazolam 1-hydroxylation) activities and repaglinide metabolism were studied using human liver microsomes. At therapeutic repaglinide concentrations (<0.4 microM), CYP2C8 and CYP3A4 metabolised repaglinide at similar rates. Quercetin (25 microM) and itraconazole (3 microM) inhibited the metabolism of 0.2 microM repaglinide by 58% and 71%, and that of 2 microM repaglinide by 56% and 59%, respectively. The three fibrates inhibited CYP2C8 (Ki: bezafibrate 9.7 microM, gemfibrozil 30.4 microM and fenofibrate 92.6 microM) and repaglinide metabolism (IC50: bezafibrate 37.7 microM, gemfibrozil 111 microM and fenofibrate 164 microM), but had no effect on CYP3A4. Rifampicin inhibited CYP2C8 (Ki 30.2 microM), CYP3A4 (Ki 18.5 microM) and repaglinide metabolism (IC50 13.7 microM). In conclusion, both CYP2C8 and CYP3A4 are important in the metabolism of therapeutic concentrations of repaglinide in vitro, but their predicted contributions in vivo are highly dependent on the scaling factor used. Gemfibrozil is only a moderate inhibitor of CYP2C8 and does not inhibit CYP3A4; inhibition of CYP-enzymes by parent gemfibrozil alone does not explain its interaction with repaglinide in vivo. Rifampicin competitively inhibits both CYP2C8 and CYP3A4, which can counteract its inducing effect in humans.