The ovarian responses to gonadotropic principles have not been the same in any two species of animals investigated. Varying degrees of luteinization of mature follicles with no constancy of ovulation have been observed in the immature mouse following injections of these principles. In the ovaries of the immature rat after similar injections there occurred cystic follicles with little evidence of luteinization. In the rabbit there were cystic and often hemorrhagic follicles. Intravenous injections of these principles produce ovulation in the mature rabbit. Corpora lutea were not produced in the ovaries of the immature Macacus monkey by the use of a single extract, but Engle (1) observed their occurrence following subcutaneous injections of a gonadotropic extract prepared from the anterior lobe of the pituitary followed by the intravenous administration of this extract and a similar one from pregnancy urine. Studies of the ovaries of the human (2) following the use of pregnancy urine extract and after a gonadotropic principle of anterior pituitary have been described previously. Observations (3) have been reported also on the endometrium in socalled ‘endometrial hyperplasia’ after treatment with pregnancy urine extracts, with a gonadotropic principle of the anterior lobe and with other principles. We have re-examined recently all the material accumulated to date, made additional investigations and have endeavored to correlate clinical and tissue responses to socalled gonadotropic therapy. These studies were the basis for an analysis (4) of the therapeutic possibilities of these principles. They led also to a summary by one of us (5) of the responses of the human gonads to anterior pituitary-like principles. In reviewing this material we have encountered and corrected the diagnostic errors which occurred in several of the previous reports.