Using a highly specific radioimmunoassay we have measured immunoreactive human growth hormone releasing factor (ir-hGRF) concentrations in the peripheral circulation of a total of 12 normal subjects. Neither insulin-induced hypoglycaemia, intravenous arginine nor oral carbohydrate caused any change in venous plasma ir-hGRF concentrations, despite the expected stimulation and suppression respectively of growth hormone secretion. Growth hormone secretion was not increased by oral fat or protein but each of these two foods stimulated ir-hGRF concentrations two- to four-fold. Spontaneous pulses of growth hormone secretion on control days were unaccompanied by any increase in plasma ir-hGRF. The dissociation between peripheral circulating ir-hGRF and growth hormone responses demonstrated under different circumstances suggests that an important source of human growth hormone releasing factor lies outside the hypothalamus and that secretion from this source is unconnected with the normal control of pituitary growth hormone secretion.