The in vivo chronic infusion of growth hormone releasing factor (GRF) results in a loss of the pituitary growth hormone (GH) response to GRF as well as in a substantial depletion of pituitary GH content. To evaluate if the loss in response is due to the down-regulation of GRF receptors the specific GRF binding capacity of pituitary homogenates prepared from rats infused with saline or GRF (1 or 15 µg/h for 24 h) was determined. The pituitary binding capacity of animals infused with GRF was significantly reduced as compared to animals infused with saline