The acute effects of human growth hormone (GH) on the basal levels of glucose and insulin in blood were investigated in 11 healthy men. GH doses of 5, 10, 20, and 40 mug/kg body weight were given iv as a constant-rate infusion over 30 min, and resulted in peak hormone levels (30 min) of 20.5 plus or minus 1.0, 48.5 plus or minus 2.2, 108.2 plus or minus 4.5, and 229.2 plus or minus 14.6 ng/ml, respectively. There was a small (max 9.8 plus or minus 2.6%) but significant decrease in the blood glucose level, observed already at 15 min after the beginning of the GH infusion and persisting up to 90 min. The highest dose of GH induced the most marked changes, but there was otherwise no clear correlation between dose and effect. The basal plasma insulin levels showed a more marked (max 16.0 plus or minus 4.7%) decrease which was not correlated, in time or in magnitude, with the changes in blood glucose. In some subjects, in whom no significant decrease in blood glucose was observed, plasma insulin still demonstrated a similar fall (max 20.2 plus or minus 7.6%). Neither were these changes in plasma insulin correlated to the dose of GH within the range used in this study. The findings suggested that the early, insulin-like effect of GH on blood glucose is distinct from its effect on the pancreas. The latter is a suppressive one, consistent with earlier findings on glucose-induced insulin release.