Peripheral plasma concentrations of GH in adult chickens were increased, in a dose-related manner, between 5 and 30 min after the intracerebroventricular (i.c.v.) injection of 0.1 or 10 micrograms TRH. In contrast, i.v. administration of comparable doses of TRH had no significant effect on circulating GH concentrations. [3H]3-methyl-histidine2-TRH [( 3H]Me-TRH) was located in the pituitary gland and peripheral plasma within 5 min of its i.c.v. administration, although in amounts that were unlikely to affect directly pituitary function. [3H]Me-TRH rapidly accumulated in the hypothalamus following its i.c.v. administration (but not after i.v. injection), and the central effect of TRH on GH secretion in birds is therefore likely to be induced by effects at hypothalamic sites.