Cytochromes P450 (CYP) 3A4 is the most abundant human hepatic CYP isoform catalyzing the metabolism of approximately 50% of therapeutic agents. In addition to inhibition or induction, CYP3A4 is subject to stimulation, termed homotropic (substrate stimulation) and heterotropic (stimulation by effectors) cooperativity. The heterotropic cooperativity of CYP3A4 may result from an increase in Vmax, a decrease in Km or a combination of the two and sometimes exhibits regio-selectivity when the enzyme is involved in two or more metabolic pathways for a single substrate. An effector of CYP3A4 can also be a substrate; its metabolism may or may not be inhibited by another substrate. These characteristics of heterotropic cooperativity of CYP3A4 have been interpreted in the context of two binding domains in the active site of the enzyme, two substrate binding plus a distinct allosteric binding site, multiple enzyme conformations or multiple binding sites accompanied by conformational changes. Examples of in vivo CYP cooperativity are rare; representative cases include flavone-dependent stimulation of zoxazolamine metabolism in rats and enhancement of CYP3A-mediated hepatic clearance of diclofenac by quinidine in monkeys. Effector-induced increases in CYP3A4 activity were observed during the 1'-hydroxylation of midazolam and 4'- and 10-hydroxylation of warfarin in human hepatocyte systems. These data imply that CYP cooperativity has the potential to cause in vivo drug-drug interactions. Because cooperative and inhibitory responses from CYP3A4 are known to be substrate-dependent, projection of the pharmacokinetics of an investigational drug and CYP-associated risks of drug-drug interactions in humans can be very complex. Further investigation of CYP cooperativity is warranted.