Progesterone and Estrogen in the Experimental Control of Ovulation Time and Other Features of the Estrous Cycle in the Rat

introduction Several years ago it was reported that small daily doses of progesterone given to persistent-estrous rats serve to interrupt the equilibrium between hypophysis and ovaries, inducing sequences of vaginal cycles which closely resemble those in normal animals (Everett, 1940). In a significantly high proportion of these cycles ovulation occurs and new corpora lutea are formed. This finding cannot be explained on the assumption which has been generally held for many years that the principal effect of progesterone upon the hypophysis is suppression of LH secretion. Subsequent studies have made it more and more apparent that under certain conditions progesterone facilitates the release of luteinizer. In 1943 we reported that if persistent estrus is interrupted experimentally by a single injection of progesterone, a diestrous interval of 2 to 3 days usually occurs followed by the typical proestrous smear picture and then full cornification which continues if no further treatment is given.