The influence of experimental hyperinsulinemia on energy intake, energy expenditure, and body composition was investigated in rats treated chronically with high doses of insulin. Energy balance studies, each of 2-wk duration, were conducted with two different long-acting insulins (Protamine and Monotard), administered in the morning (IM), the late afternoon (IA), or both (IMA) and in animals of three different ages, namely in 4-, 8-, and 12-wk-old rats. The results indicate that the level of hyperphagia induced by insulin was markedly influenced by the type of long-acting insulin (P less than 0.001; Protamine greater than Monotard), by age (P less than 0.001; 12 greater than 8 greater than 4 wk), as well as by the timing of insulin administration (P less than 0.002, IMA greater than IM or IA). Body protein deposition was unaltered, but body fat and energy expenditure increased in parallel to the level of hyperphagia. Regression analysis shows a strong linear correlation (r = 0.963) between the change in energy expenditure and the change in energy intake in response to insulin and indicates that approximately 50% of the excess calories consumed was dissipated as heat. In the absence of hyperphagia, however, insulin administration had no effect on energy expenditure nor on energy partitioning. Similarly, the influence of altered meal pattern, induced by administering insulin at different times of the day, was also found to have no impact on energy expenditure. The current investigations therefore refute the notion that high doses of insulin via hyperinsulinemia and/or altered meal pattern have an inhibitory influence on whole body thermogenesis. In contrast, our data demonstrate that the adaptive phenomenon that tends to minimize the accumulation of excess caloric intake, i.e., diet-induced thermogenesis, persists in the hyperinsulinemic state.