Single iv doses (25, 50 and 100 micrograms) of medetomidine, a selective alpha 2-adrenoceptor agonist of the imidazole type, were administered to 8 healthy male volunteers in a randomized, double-blind, placebo-controlled study. The concentration of hGH in plasma was powerfully and dose-dependently increased. The plasma level of cortisol was dose-dependently decreased, whereas TSH showed a slight but statistically significant increase. Plasma levels of PRL, FSH and LH were unaffected by the drug. Medetomidine appears to resemble other alpha 2-adrenoceptor agonists, notably clonidine, in its endocrine effects. Its high selectivity and short duration of action make it a suitable tool for studies of the physiology and pharmacology of alpha 2-adrenoceptors in man.