The action ofhydroxyprogesterone capronate on the muscular activity ofthe cranial, medial and caudal segments of the right and left uterine horns of 15 primiparous Guinea pigs (Cavia porcellus) at 10-12 days gestation was examined. Additive doses of progesterone were given until amaximumresponse of23.813xl0-7 mM was achieved. The percentage of subsequent muscular relaxation of both horns was measured in relation to maximum muscular tension (determined by depolarization of the membrane with KCl). The greatest degree of muscular relaxation occurred in the cranial (104.23±±2.89 and 127.06±±3.09 for the left and right horns, respectively) and caudal (110.41±±3.18 and 70.14±±2.18 for the left and right horns, respectively) segments of the uterus. Although lesser values (P<0.05) were found for the medial segment (54.45±±2.24 and 50.82±±1.72 for the left and right horns, respectively), the greatest proportion of embryos were implanted here (35.42% and 25.00% in the left and right horns, respectively), suggesting that the progesterone has not yet established its inhibitory role over muscular activity.