The effect of experimentally induced hyperinsulinemia on body composition was studied in rats with food intakes precisely controlled by intragastric feeding and physical activity manipulated by sedation with chlordiazepoxide (CDP). Male Sprague-Dawley rats (n = 38) were fitted surgically with gastrostomy tubes. After 8 days the animals were divided into four groups. Group 1 received daily injections of protamine-zinc insulin; group 2 received daily injections of saline; group 3 received the same insulin doses as group 1 plus daily administration of CDP mixed with the diet; group 4 received daily injections of saline plus CDP in the diet. All groups were tubefed identical amounts of semiliquid diet via gastrostomy. Physical activity was measured by electronic monitor. After 4 wk the rats were killed. The insulin-treated groups (1 and 3) had significantly larger fat depots and larger mean fat cell size than the noninsulin-treated groups (2 and 4). This increase in fat occurred concurrently with a decrease in carcass protein and water. Physical activity, as measured, was unaltered by insulin but was significantly reduced by CDP. Treatment with CDP only increased the dorsal fat depot and liver weight but had no significant effect on total dissected fat depots and had a reductive effect on carcass protein. In conclusion insulin treatment enhanced the efficiency of conversion of energy intake into fat energy stores.