A single oral dose of clonidine (0.15 mg), a selective alpha-adrenergic stimulating agent, was able to increase plasma growth hormone (GH) levels (above 5 ng/ml) in 6 out of 7 normal men tested. This GH increase was independent of the hypotensive effect of the drug and was observed without any modification of plasma prolactin, thyroid stimulating hormone, gonadotropins and glycemia. When oral clonidine administration was associated to a slow thyrotropin releasing hormone (TRH) infusion (1 mg dissolved in 400 ml of 0.9% saline solution, at a constant rate during 150 min) the plasma GH response was significantly inhibited when compared with that observed after clonidine alone. These results suggest that in normal subjects TRH is capable of blocking the effect of an alpha-adrenergic stimulus which is conceivably acting at the level of central nervous system.