A series of androst-5-en-7-ones and androsta-3,5-dien-7-ones and their 7-deoxy derivatives, respectively, were synthesized and tested for their abilities to inhibit aromatase in human placental microsomes. All the steroids inhibited the enzyme in a competitive manner with Ki's ranging from 0.058 to 45 microM. The inhibitory activities of 17-oxo compounds were much more potent than those of the corresponding 17 beta-alcohols in each series. Steroids having an oxygen function (hydroxy or carbonyl) at C-19 were less potent inhibitors than the corresponding parent compounds having a 19-methyl group. 3,5-Dien-7-one 24 and its 19-hydroxy and 19-oxo derivatives (12 and 13) as well as 19-oxo-5-en-7-one 3 caused a time-dependent inactivation of aromatase only in the presence of NADPH in which the kinact values of 19-als 3 and 13 (0.143 and 0.189 min-1, respectively) were larger than those of the corresponding 19-methyl (23 and 24) and 19-hydroxy (1 and 12) steroids, respectively. 19-Nor-5-en-7-one 4 but not its 3,5-diene derivative 14 also inactivated the enzyme in a time-dependent manner. In contrast, 7-deoxy steroids 21 and 27, having a 19-methyl group, did not cause it. The inactivations were prevented by the substrate androstenedione, and no significant effects of L-cysteine on the inactivations were observed in each case. The results suggest that oxygenation at C-19 would be at least in part involved in the inactivations caused by the inhibitors 23 and 24. The conjugated enone structures should play a critical role in the inactivation sequences.