Female rhesus monkeys were treated with exogenous human menopausal gonadotropin (Pergonal) in the presence of exogenous systemic progesterone provided by Silastic implants and during the luteal phase of the menstrual cycle to determine if progesterone influences gonadotropin-induced follicular maturation. Peripheral serum and ovarian vein plasma 17 beta-estradiol concentrations were used to assess the response to gonadotropin treatment. When systemic levels of progesterone were elevated to 6--10 ng/ml in eight animals during the midfollicular phase of the menstrual cycle, each animal responded to exogenous gonadotropin with a sustained increase in serum 17 beta-estradiol concentration, which reached levels of approximately 800 pg/ml during treatment. To determine if the corpus luteum itself influences the ovarian response to exogenous gonadotropin, animals were treated with exogenous gonadotropin for 7 days, commencing during either the follicular phase of the cycle or the midluteal phase of the cycle. Regardless of the time of initiation of gonadotropin treatment, peripheral serum 17 beta-estradiol levels rose to approximately 1 ng/ml in all animals studied. 17 beta-Estradiol levels were elevated in ovarian venous blood from the ovary, which contains the active corpus luteum and the contralateral ovary. These observations demonstrate that exogenous gonadotropins can stimulate follicular maturation in the presence of progesterone.