The relationship between food intake and thyroid function has been investigated in immature domestic fowl. Starvation delayed, but did not suppress, the triiodothyronine (T3) response to intravenously administered thyrotropin-releasing hormone (10 micrograms/kg). This probably resulted from a suppression of monodeiodinase activity, since the conversion of thyroxine (T4) to T3 in thyroidectomised birds following an intramuscular injection of T4 (10 micrograms/kg) was markedly reduced by starvation. Starvation, for 24 or 48 hr, lowered the circulating T3 level but increased the T4 concentration. When fasted birds were refed the T4 concentration was initially enhanced but subsequently declined as the T3 concentration progressively increased. The accompanying decline in the T4:T3 ratio in fasted-refed birds indicated that the rise in the T3 level resulted from the peripheral monodeiodination of T4. The increase in T3 concentration could be induced solely by carbohydrate; the intraperitoneal administration of glucose (2.0 g/kg) to fasted birds resulting in a slight, transient rise in the T3 concentration and a fall in the T4:T3 ratio. The generation of T3 was also energy dependent, in that the magnitude of the T3 response of fasted birds to refeeding was proportional to the amount of food consumed and to the metabolisable energy (ME) content of the diet. Moreover, when exogenous T4 (100 micrograms/kg) was intramuscularly administered to thyroidectomised birds fed a diet with a high ME content, the conversion of T4 to T4 was greater than that in birds fed a diet of lower ME content. These results demonstrate that nutritional stimuli are involved in the regulation of thyroid function in birds, particularly in the peripheral generation of T3.