Exemestane (6-methylenandrosta-1,4-diene-3,17-dione; FCE 24304) is an orally active irreversible aromatase inhibitor which is in phase II clinical evaluation for the potential therapy of postmenopausal breast cancer. A series of exemestane analogs, with modifications at the 6-methylene group and with additional reduction at the 17-keto group, were synthesized as potential metabolites and tested in vitro for their effect on human placental aromatase. All these new analogs were found to be less potent in inhibiting aromatase than exemestane. The most effective compound was the 17 beta-hydroxy-derivative (compound 2), which is 2.6-fold less potent than exemestane [50% inhibitory concentration (IC50) 69 and 27 nM, respectively]. The various C-6 modified derivatives of the 17-oxo series were found to inhibit the aromatase enzyme in the following descending order: 6-methylene (exemestane) > 6-spirooxirane (6) > 6 beta-hydroxymethyl (11) > 6-hydroxymethyl (7) > 6 beta-carboxy (13), showing IC50 values of 27, 206, 295, 2,300, and 7,200 nM, respectively. The 17 beta-hydroxy analogs of some of the above mentioned compounds were also synthesized (3,4,12) and found to be 3-8-fold less potent than the corresponding 17-keto analogs.