The pituitary reserve of GH [growth hormone] prolactin TSH [thyrotropin], LH [luteinizing hormone] and FSH [follicle-stimulating hormone] was studied in a group of 13 acromegalic patients with the aim of evaluating the pituitary function and the activity of the disease. Plasma GH, TSH and prolactin were determined after thyrotropin releasing hormone (TRH) administration, plasma gonadotropins and GH after luteinizing hormone releasing hormone (LH-RH) administration. The plasma TSH response to TRH was generally blunted in the patients treated with pituitary irradiation; none of the patients with diminished TSH reserve had signs of hypothyroidism. Six acromegalics showed prolactin basal levels higher than controls; none had galactorrhea; 4 complained of impairment of the gonadal function. The prolactin response to TRH was variable and not related to prolactin basal levels. A subnormal LH reserve after LH-RH stimulation was observed in 5 of 10 patients; 4 of them had clinical signs of hypogonadism. A normal FSH response to LH-RH was present in all patients. A nonspecific GH response to TRH and or LH-RH was obtained in about half of the acromegalics studied. GH responsiveness to TRH and/or LH-RH was not related to the activity of the disease or to a specific derangement of the hypothalamo-pituitary function. A concordant response was observed only between GH and prolactin response to TRH; the highest prolactin responses to TRH were obtained in the GH responsive patients. Each patient showed a constant GH pattern of response on repeated testing. Even after pituitary irradiation the pattern of GH response was unchanged in spite of lowered GH plasma levels.