14,15-Secoestra-1,3,5(10)-trien-15-yne-3,17β-diol (1) is a mechanism-based inactivator of human placental 17β,20α-hydroxysteroid dehydrogenase (estradiol dehydrogenase, EC 220.127.116.11). Inactivation with alcohol 1 requires NAD-dependent enzymic oxidation and follows approximately pseudo-first-order kinetics with a limiting t1/2 of 82 min and a “Ki” of 2.0 μM at pH 9.2 and 25 °C. At saturating concentrations of NAD, the initial rate of inactivation is slower than in the presence of 5 μM NAD, suggesting that cofactor binding to free enzyme impedes the inactivation process. Glutathione completely protects the enzyme from inactivation at both cofactor concentrations. Inactivation with 45 μM tritiated alcohol 1 followed by dialysis and gel filtration demonstrates a covalent interaction and affords an estimated stoichiometry of 1.4 molecules of steroid per subunit (2.8 per dimer). Chemically prepared 3-hydroxy-14,15-secoestra-l,3,5(10)-trien-15-yn-17-one (2) rapidly inactivates estradiol dehydrogenase with biphasic kinetics. From the latter phase, a Ki of 2.8 μM and a limiting t1/2 of 12 min at pH 9.2 were determined. Estradiol, NADH, and NAD all retard this latter inactivation phase. We propose that enzymatically generated ketone 2 inactivates estradiol dehydrogenase after its release from and return to the active site of free enzyme.