Aromatase, the important regulatory enzyme that converts androgens to estrogens, is found in relatively high levels in the human placenta. However, since the ovary is the major source of the estrogens in females, we undertook studies to compare the rodent ovarian enzyme with that from human placenta. Pregnant mare's serum gonadotropin (PMSG) markedly increases aromatase activity in the ovaries of immature rats, and this model was used in order to reproducibly obtain high enzyme levels. An injection of PMSG resulted in a specific stimulation of aromatase activity 12 times the increase in ovarian weight in 48 h. Kinetic studies demonstrated that, although the PMSG-stimulated ovarian microsomes had one-tenth the specific activity of the human placenta, the Km values were similar (about 33 and 44 nM, respectively). The potent inhibitor of placenta aromatase, 10-propargylestr-4-ene-3,17-dione, was used to further characterize the enzyme. It inhibited the rat aromatase with an I50, of 36 nM and exhibited time-dependent inhibition with a half-life of inactivation of 16 min and a Ki of 15 nM. These values are similar to those we obtained with the human enzyme (10 nM, 12 min, and 5nM, respectively). The enzyme parameters in the presence and absence of the inhibitor suggest that the enzymes from the two sources are kinetically quite similar.