The effect of prolonged administration of synthetic LH-RH (400 mug daily im for 5 days) on gonadotropin response to LH-RH (100 mug single injection iv) was compared in 13 patients with pituitary tumors or suprasellar tumors. No patients with pituitary tumors exhibited an augmented response on serum FSH and LH after prolonged LH-RH administration. A paradoxical fall in response to LH-RH test occurred in 3 patients with pituitary tumors and in a patient with hypothalamic tumor in whom gonadotropin responses were normal on the first LH-RH test. On the other hand, 2 patients with suprasellar tumors with a low response to the initial LH-RH test showed significant increase on a second LH-RHtest after consecutive LH-RH administration. Consecutive administration of LH-RH may help to distinguish between the hypogonadism of pituitary origin and that of hypothalamic origin when the initial LH response to a single dose of LH-RH is subnormal.