Ovarian α—hydroxysteroid dehydrogenase (α—OH—SDH) activity and serum and pituitary concentrations of luteinizing hormone (LH) and prolactin were determined in individual rats at selected times during the estrous cycle, normal pseudopregnancy, prolonged pseudopregnancy due to hysterectomy or induced deciduomata, and pregnancy. During die 5-day cycle, α—OHSDH activity was lowest at diestrus II (38.8 ± 2.5 mU enzyme/mg protein), increased 2-fold at proestrus, then declined at estrus (52.8 ± 4.8). Except for sharp peaks at proestrus, serum LH and prolactin levels were consistently low at diestrus and estrus (mean ± SE over 3 days of diestrus = 15.5 ± 1.4 ng LH/ml serum and 48.1 ± 3.1 ng prolactin/ml serum). During normal pseudopregnancy, enzyme activity on Day 1 was comparable to that found at diestrus II of the cycle, then declined 3-fold at Day 6. After Day 6, α— OH—SDH increased steadily until Day 13 (60.9 ± 4.6 mU/mg protein). From Days 1-6, enzyme levels in pregnant rats were quite similar to those in normal pseudopregnant animals. After Day 6, however, activity remained low during pregnancy until Day 21 and during prolonged pseudopregnancy until Day 17. When compared to serum LH levels during the diestrous phase of the cycle, LH levels throughout the other four reproductive states were low until the terminal days. There was a tendency for LH levels to gradually increase over the last several days of normal pseudopregnancy. Serum prolactin concentration at Day 1 of normal pseudopregnancy was about 3-fold greater than levels on diestrous days of the cycle, declined steadily to the morning of Day 13, and then increased sharply that evening. Prolactin was similarly elevated in pregnant rats until Days 4 or 6, after which levels remained low until Day 19. The concentration doubled at Day 20 and then tripled by Day 21 of pregnancy. Elevated prolactin levels during prolonged pseudopregnancy only occurred at Day 6 and proestrus, Day 21. A positive correlation existed between serum LH levels and α—OH—SDH activity which supports the contention that LH may be associated with the onset of α—OH—SDH activity during the terminal stages of pregnancy and pseudopregnancy. Since the patterns of endogenous LH and prolactin levels differed for pregnancy and pseudopregnancies, the significance of this association may be dependent upon the particular reproductive state. (Endocrinology91: 1499, 1972)