The effect of acute administration of growth hormone-releasing hormone (GHRH), clonidine (CLO), an alleged GHRH releaser, or GHRH and clonidine given simultaneously was studied in young and old dogs. Simultaneous administration of CLO induced in young dogs an additive effect on GH release and potentiated in old dogs the GHRH-induced GH release, with the GH response being clearly higher than the sum of the GH responses to GHRH or CLO alone. These data suggest that CLO promotes GH release in the dog also by inhibition of somatostatin release.