The aim of the present study was to determine whether heat acclimation increases whole-body sensitivity to insulin. Male Wistar rats were kept at 34 degrees C for 2 weeks (HA group). Warm-acclimated rats (WA group) at 25 degrees C served as controls. The glucose infusion rate (GIR) was assessed as an index of in vivo insulin sensitivity, using a hyperinsulinemic euglycemic clamp technique. Moreover, the 125I-insulin binding capacity to purified insulin receptor preparations from m. gastrocnemius of rats after 1, 7, or 14 days of heat exposure was examined. Mean GIR values of HA group were slightly higher than those of WA group, but not significant. The great deviation of the HA group, however, appeared to exist. About half of HA rats showed markedly high GIR values (p < 0.01 vs. WA group). Mean GIR value of the remaining HA rats were significantly (p < 0.05) lower than those of the WA group. Likewise, the binding capacity to 125I-insulin was not significantly different among the periods of time of heat exposure, and the deviation went on increasing from 1 to 14 days. These results suggest that there is a wide individual difference between the changes in glucose metabolism under heat exposure. In view of such results, there appears to be a great need for further studies on the factors affecting the variation of insulin action.