We conducted two experiments with preruminant calves weighing 80 to 240 kg to study the long-term nutritional regulation of circulating IGF-I, thyroxine (T4), and triiodothyronine (T3). The two experiments were similar in design but were performed with calves of two live weight ranges: 80 to 160 kg (Exp. 1) and 160 to 240 kg (Exp. 2). In each experiment, 36 calves were allocated to one of 12 dietary treatments, which consisted of six protein intake levels at each of two energy intake levels. Digestible protein intakes ranged between .90 and 2.72 g N·BW-.75·d-1 in Exp. 1 and between .54 and 2.22 g N·BW-.75·d-1 in Exp. 2. The energy intake levels were kept constant on a protein-free basis; increased energy intakes were realized by increasing energy intake from fat and carbohydrates in a fixed ratio. The digestible protein-free energy intakes were 663 and 851 kJ·BW-.75·d-1 in Exp. 1 and 564 and 752 kJ·BW-.75·d-1 in Exp. 2. Blood samples were taken 5 to 6 h after feeding once every 14 d until the calves reached their target weight. In both experiments, plasma IGF-I and T4 concentrations increased with increasing protein intake (P < .01), but they were unaffected by protein-free energy intake (P > .10). In both experiments, plasma T3 levels were markedly higher at the high protein-free energy intake level (P < .01) and increased slightly with increasing protein intake in Exp. 1 (P = .19) and Exp. 2 (P < .01). Results of these experiments suggest the involvement of IGF-I in the response of protein deposition to increased protein intakes and the involvement of the active thyroid hormone T3 in the response of protein deposition to increased protein-free energy intakes.