Preterm delivery, that is delivery before 37 completed weeks of gestation, is the major determinant of neonatal morbidity and mortality. Until recently, no effective therapies for prevention of preterm birth existed. In a recent multicentered trial, 17alpha-hydroxyprogesterone caproate (17-OHPC) reduced the rate of preterm birth by 33% in a group of high-risk women. Limited pharmacologic data exist for this drug. The recommended dose is empiric; the metabolic pathways are not well defined especially in pregnant women; and the fetal exposure has not been quantified. To define the metabolic pathways of 17-OHPC we used human liver microsomes (HLMs), fresh human hepatocytes (FHHs), and expressed enzymes. HLMs in the presence of NADPH generated three metabolites, whereas two major metabolites were observed with FHHs. Metabolism of 17-OHPC was significantly inhibited by the CYP3A4 inhibitors ketoconazole and troleandomycin in HLM and FHH. Metabolism of 17-OHPC was significantly greater in FHH treated with the CYP3A inducers, rifampin and phenobarbital. Furthermore, studies with expressed enzymes showed that 17-OHPC is metabolized exclusively by CYP3A4 and CYP3A5. The caproic acid ester was intact in the major metabolites generated, indicating that 17-OHPC is not converted to the primary progesterone metabolite, 17alpha-hydroxyprogesterone. In summary, this study shows that 17-OHPC is metabolized by CYP3A. Because CYP3A is involved in the oxidative metabolism of numerous commonly used drugs, 17-OHPC may be involved in clinically relevant metabolic drug interactions with coadministered CYP3A inhibitors or inducers.