Serum levels of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) of Macaca mulatta females were compared by radioimmunoassay after iv and intrapituitary administration of sheep (o) and rabbit (ra) median eminence extracts (SME) and synthetic LH-releasing hormone (LH-RH). Both intrapituitary infusion and iv injection of oSME and raSME caused nearly equal and significant (p < 0.05) elevations in serum levels of LH within 30 min. Intrapituitary infusion of 40 μg of LH-RH into rhesus at each of 3 stages–days 6 to 7, 11 to 14, or 20 to 23–of their menstrual cycles also caused significant, but equal, increases in serum LH. In contrast, an iv injection of 40 or 85 ng LH-RH during the early follicular or periovulatory stages of the cycle failed to elevate serum LH or FSH significantly above preinjection levels. Since the concentrations of serum LH after LH-RH never approached the LH levels observed during the spontaneous midcycle surge regardless of the LHRH dosage, its mode of administration, or the stage of the menstrual cycle at the time of administration, the relative insensitivity of the rhesus pituitary to synthetic LH-RH was not resolved. However, these data suggest that LH-RH is inactivated more rapidly when injected systemically than when infused directly into the pituitary gland. (Endocrinology93: 814, 1973)