The present study was carried out to determine if alterations in steroid-producing ability of ovary could account for the abnormal embryonic development which follows delayed ovulation. The rats treated for 1 or 2 days with Nembutal were used as the animal model with delay of ovulation. In normal rats before the ovulation, the ovaries secreted predominantly estradiol-17 beta (E2) and later, after the ovulation, produced mainly progesterone (P) and 5 alpha-dihydroprogesterone (5 alpha-DHP). In rats that failed to ovulate, the levels of E2 remained high on the first day of blockade and decreased to very low levels on the second day of blockade, while P and 5 alpha-DHP remained at low levels during this period. After the ovulation which follows delayed ovulation, there was still a shift in the steroid secretory patterns, which was similar to that observed in the normal ovaries. However, the concentrations of 5 alpha-DHP was significantly lower than that after normal ovulation. The injection of LH greatly increased the secretion of 5 alpha-DHP within 30 min in both the normal and the blocked ovaries, but the steroid-producing ability of ovaries which follows delayed ovulation was low, suggesting low 5 alpha-reductase in the ovary with delayed ovulation. It is said that physiologic effect of 5 alpha-DHP is concerned in life of blastocysts. These results suggest that the abnormal embryonic development which follows delayed ovulation may be causally related to the low 5 alpha-reductase activity in the blocked ovaries.