Glucocorticoids have been shown to inhibit GH secretion in normal man when administered in large amounts for several days. The aim of our study was 1. to investigate the acute effects of a single dose of glucocorticoids on GH secretion in normal man; 2. to look at the relationship between the increase in serum cortisol concentration and GH response to the stimuli. Six healthy volunteers received on three occasions in random order an iv injection of GHRH (1-29) NH2, 100 micrograms, alone or 60 min after oral administration of either 25 or 50 mg of cortisone acetate. Mean stimulated GH levels, GH peak and integrated GH concentration were significantly lower after GHRH plus cortisone 25 mg than after GHRH alone. Mean GH levels at 15 and 30 min after GHRH injection and the peak GH level showed a further decrease after GHRH plus cortisone 50 mg. We conclude that acute administration of pharmacological doses of glucocorticoids is able to inhibit GH response to GHRH, probably through enhancement of endogenous somatostatin release. Moreover, this pharmacological effect of glucocorticoids seems to be dose-dependent and thus directly related to serum cortisol concentrations.