Twenty-eight-day old broiler chicks were used to determine growth rate and thyroid activity responses to various doses and routes of administration of synthetic thyrotropin-releasing hormone (TRH). In experiment 1, daily intramuscular (im.) injections of 2 µg TRH/chick over 10 or 20 days stimulated the relative body weight gain in cockerels by 5.7 and 10%, respectively, compared with saline injected controls. In experiment 2, TRH administered im. (2 µg/chick) or orally (80 µg/chick) over 10 days significantly increased (P≤0.01) the relative body weight gain of cockerels by 20.3 and 29.6%, respectively and slightly improved feed conversion efficiency. No effect of TRH on the growth rate of female chicks was found. Treatment with TRH induced a marked increase of relative thyroid size, basal metabolic rate and plasma triiodothyronine and thyroxine levels. Our data confirmed earlier reports that improvement in growth rate of TRH-treated chicks appears to be mediated by thyroid hormones.