The effects of intramuscularly and orally administered medroxyprogesterone acetate on cytochrome P450 3A4 (CYP3A4) activity were investigated in twelve postmenopausal women in a randomized, crossover study. Unbound prednisolone clearance and the erythromycin breath test were used as markers of CYP3A4 activity. After 2 months of intramuscular progestin therapy, unbound prednisolone clearance increased by 25% in five of six subjects. Similarly, after intramuscular progestin therapy, results from the erythromycin breath test showed a 23% mean increase in CYP3A4 activity. In contrast, 2 months of oral progestin therapy had no effect on prednisolone pharmacokinetics or erythromycin metabolism. These results suggest that parenterally but not orally administered progestins may induce or activate the CYP3A4 enzyme system, leading to an increased metabolism of many CYP3A4 substrates.