Treatment of pseudopregnant rats with ergocryptine mesylate (ECR) depressed serum pro-laetin levels but had no apparent effect on LH secretion. Ovarian progesterone secretion was significantly reduced 24 hr after ECR treatment on day 6 or 9 of pseudopregnancy and the secretion rate of 20α-OH-P remained constant. When lactating rats nursing 6 pups were treated with ECR on day 6 or 9 postpartum, progesterone secretion was significantly decreased by 48 hr after treatment and 20α-OH-P secretion was not altered. Furthermore, ECR inhibited litter weight gains of these lactating dams. After treatment of normal pregnant and pregnant lactating rats with ECR on day 6 of pregnancy, gestation was terminated in all animals. If ECR was given on day 9 to normal pregnant rats, t o pregnant lactating animals whose pups were removed on day 9 postpartum, or to pregnant lactating rats treated with LH, gestation was not terminated. However, treatment with ECR of day 9 pregnant lactating rats whose pups continued to suckle terminated pregnancy in 13 of 21 animals. The results of these studies suggest that elevated pituitary prolaetin secretion is necessary for maintenance of luteal function in pseudopregnant and lactating rats, and in pregnant lactating rats elevated pituitary prolaetin secretion is a component of the luteotrophic cornplex for a longer period of time than in normal pregnant rats. (Endo-crinology96: 335, 1975)