Although the increased sensitivity of hypertensive animals to heat stress has been reported, the effect of chronic heat exposure has not been examined. The specific goal of the present investigation was to study the impact of chronic heat treatment on the blood pressure of spontaneously hypertensive mice. Chronic 40 degrees C heat exposure for 5 min daily progressively lowered basal blood pressure in hypertensive mice within 20 days, without any change in normal mice. In fact, after 35 days of chronic heat treatment, the basal blood pressure of hypertensive mice was indistinguishable from that of the normotensives. Repeated immobilization and prewarming as normal procedures for recording blood pressure contributed to the decrease in blood pressure by 10 to 12 mm Hg, but chronic heat by itself was significantly more potent in reducing it by an additional 20 mm Hg. After the discontinuation of chronic heat application, the basal blood pressure of hypertensive mice returned with time to the level registered in sham-handled hypertensive controls. These results demonstrate that, although acute heat is more detrimental to hypertensive mice, brief, chronic exposure to mild heat stress is beneficial in that it normalizes basal blood pressure.