Groups of normal adult male rats were rapidly decapitated at 3 h intervals for radioimmunoassay determination of pituitary growth hormone (GH) content and plasma GH concentration. Although both pituitary and plasma GH levels varied markedly with the time of day, only the latter variations were statistically significant. The periodicity in plasma GH was bimodal, with the nadir occurring approximately 2 h after the onset of the dark phase of the light-dark cycle. Ether and immobilization stress produced a marked reduction in plasma GH and abolished the rhythm in circulating GH; novelty stress failed to suppress plasma GH throughout the 24 h period. These data indicate that GH secretion in the rat fluctuates with a predictable 24 h periodicity and is sensitive to environmental change.