The ability of triiodothyronine (T3) to reduce basal and secretagogue-induced growth hormone (GH) release was examined in anesthetized young and adult male chickens. Infusion of T3 had no effect on basal plasma concentrations of GH in either young or adult chickens. However, GH secretion following challenge with either thyrotropin-releasing hormone (TRH) or growth hormone-releasing hormone (GRF) was reduced, in a dose-dependent manner, by the infusion of T3. In vivo sensitivity to T3 inhibition was greater with TRH- than GRF-stimulated GH release in either young (ED50 for TRH-induced GH release, 0.34 microgram T3/kg/min; ED50 for GRF-induced GH release, 0.49 microgram T3/kg/min) or adult chickens (ED50 for TRH-induced GH release, 0.11 microgram T3/kg/min; ED50 for GRF-induced GH release 1.89, micrograms T3/kg/min). Moreover, there was an increase in sensitivity of TRH-induced GH release to T3 with age.