The ability of human pancreatic GH releasing factor 1-40 (hpGRF 1-40) to release GH has been studied in rat anterior pituitary cells in primary culture. Over 24 hours hpGRF (1-40) increased total (cell content and secretion) production 2-fold with an ED50 of 20 pM. Subsequent hpGRF (1-40) stimulation of GH release was not affected by pretreatment when the fall in stored GH was taken into account. In contrast LH responses to gonadotrophin releasing hormone (GnRH) were markedly desensitized after 24 hours GnRH pretreatment in the same experimental system and using the same analysis. hpGRF (1-40) responses were not desensitized when pretreatment was for 3, 12 or 24 hours. The data show that hpGRF (1-40) responses do not desensitize in our experimental conditions under which GnRH responses show marked desensitization.