The urinary excretion pattern of 2-hydroxyestrone, estradiol, estrone, and progesterone was examined in rats during early, mid, and late pregnancy. Progesterone increased from early to mid pregnancy and declined significantly 2 to 3 days prior to parturition, corresponding to changes observed in blood levels by others. 2-Hydroxyestrone, the major estrogen in rat urine, increased significantly 4 days prior to delivery and remained elevated until it further increased sharply the day of parturition. Urinary estradiol and estrone levels showed little change until the day of parturition, when they increased significantly. Multiple correlation analysis of the data implied that 2-hydroxyestrone and estradiol were negatively correlated at the time of implantation. The results suggest that catechol estrogens, through their effect on prostaglandin synthesis, may participate in the process of implantation as well as in the mechanism involved in the onset of labor.