Human growth hormone (hGH) was administered to chronically cannulated male rats and its effect upon the physiological secretory patterns of rat growth hormone (rGH) and prolactin were observed. In comparison with injected control animals, a reduction in the size of spontaneous secretory bursts of rGH was apparent when hormone concentrations were compared 3-6 h after administration of hGH (136.27±27.31 (S.E.M.) v. 76.22±20.98 ng/ml respectively). However, the mean frequency of secretory episodes of rGH was unaltered. It is therefore postulated that endogenous rGH may modulate the amplitude but not the rhythmicity of secretory episodes of rGH.