1. In the stallion, estrogens were synthesized and sulfated in vivo by the testis. 2. The equine testicular enzyme aromatized androgens and 19-norandrogens with similar velocity, but not 16 alpha-hydroxytestosterone or epitestosterone in contrast to the human placental aromatase. 3. One single enzyme was implicated in the aromatization of androstenedione, testosterone, 19-norandrostenedione and 19-nortestosterone by ETMES. 4. During the process of androstenedione aromatization by ETMES, 19-hydroxyandrostenedione and 19-oxoandrostenedione were released and 4-hydroxyandrostenedione was a competitive inhibitor causing an additional irreversible enzyme inactivation which is what occurs with HPMES. 5. Dihydrotestosterone was a potent competitive inhibitor of aromatase activity.