The effects of (4R)-5,10-secoestra-4,5-diene-3,10,17-trione (I) and of (4R)-5,10-seco-19-norpregna-4.5-diene-3, 10,20-trione (II) on epididymal Δ4-5α-reductase and 3α-hydroxysteroid dehydrogenase were investigated. Assessment by I50 values showed that Δ4-5α-reductase was inhibited approximately 50 and 250 times more effectively than 3α-hydroxysteroid dehydrogenase by compound I and II respectively. The onset of the inhibition of Δ4-5α-reductase by these compounds occurs in less than five minutes. Both compounds were shown to be non-competitive inhibitors of Δ4-5α-reductase. For Δ4-5α-reductase the KI's for compounds I and II were 5.47 × 10-6 and 0.98 × 10-6 respectively. The results of equilibrium dialysis experiments suggested that the observed Δ4-5α-reductase inhibition was irreversible. The relative specificity of these compounds with respect to inhibitor structure and enzyme inhibition is discussed.