The effects of heat exposure and type of diet on the insulin secretory response to glucose and glucose disposal in response to insulin action in female sheep were investigated employing hyperglycemic and euglycemic clamp techniques. Animals were divided into concentrate and roughage diet groups, and were maintained at the same intake levels of metabolizable energy and crude protein in both diets. Each diet group was subjected to either thermoneutral (20 degrees C, 70% RH) or hot (30 degrees C, 70% RH) environment, followed by glucose clamp experiments. Heat-exposed sheep showed significant increases in respiration rates (P < .001) and rectal temperature (P < .05). Plasma glucose concentrations in the basal conditions were lower (P < .01) in the hot environment than in the thermoneutral environment, but there was no significant difference in basal levels of plasma insulin between the environmental treatments. In the hyperglycemic clamp experiment, mean plasma insulin increments increased (P < .05) during the heat exposure period across diet treatments. The ratio of mean plasma insulin increment to glucose infusion rate tended to be higher (P < .07) in the hot environment than in the thermoneutral environment, but diet treatment did not affect the ratio of mean plasma insulin increment to glucose infusion rate. The euglycemic clamp technique showed that glucose infusion rates remained unchanged among treatments. Insulin secretion response to glucose could be stimulated in the hot environment.