Eight weeks of heat exposure (34 +/- 0.5 degrees C) in sham-orchiectomized rats leads to an increase of body temperature, slowing of body growth rate, and decrease of serum corticosterone level, as compared with animals maintained at 21 +/- 2 degrees C. Orchiectomy decreases body temperature, slows growth rate, and increases plasma corticosterone concentration both in control and heat exposed animals. Testosterone administration reverts these parameters to initial values. We conclude that testosterone plays a role in the regulation of heat balance in male rats.