Grapefruit juice is well documented to cause clinically significant increases in the plasma concentrations of many therapeutic agents. These interactions are believed to be mediated via inhibition of intestinal cytochrome P-450 3A4 (CYP3A4) by flavonoids and/or other chemicals in grapefruit juice, although the mechanism of that inhibition has not been fully characterized. Like grapefruit juice, red wine contains large amounts of flavonoids and other xenobiotics which could also mediate CYP3A4 inhibition. In this study, we investigated the mechanism of inhibition of CYP3A4 by grapefruit juice and also examined the ability of red wine to inhibit this enzyme. Both red wine and grapefruit juice potently inhibited CYP3A4 activity in a concentration-dependent manner. At 8% of natural strength, enzyme activity was inhibited almost 90 and 84%, respectively, by grapefruit juice and red wine. In contrast, white wine did not appreciably inhibit CYP3A4 activity. Grapefruit juice irreversibly inactivated CYP3A4 in a time- and NADPH-dependent manner. The rate of inactivation mediated by grapefruit juice was similar to that mediated by troleandomycin, a potent mechanism-based inhibitor of CYP3A4. Red wine also inactivated CYP3A4 but at a rate approximately 16% that of grapefruit juice. Inhibition of CYP3A4 by red wine is primarily reversible in nature. The clinical implications of this research are discussed.