Kinetic evidence is presented for a time-dependent decrease in human placental aromatase activity by enzyme-generated intermediates derived from two widely used steroids previously described as competitive inhibitors of estrogen biosynthesis. Thus, 4-androstene-3,6,17-trione binds to the enzyme with an apparent Ki of 0.43 microM and has a pseudo-first order overall rate constant for decrease in activity of 4.03x10(-3)sec-1, while 1,4,6-androstatriene-3,17-dione has an apparent Ki of 0.18 microM and a pseudo-first order overall rate constant for decrease in activity of 1.10x10(-3)sec-1. These findings imply that the potent inhibition of estrogen biosynthesis caused by these steroids results primarily from a decrease in enzyme activity caused by enzyme-generated intermediates from the parent steroids.