Objective To investigate the effect of heat exposure during fixed time on the thoracic aorta contractile properties of rats and the related mechanism. Methods Male Wistar rats were randomly divided into control group, random-time heat exposure group and fixed-time heat exposure group (ambient temperature to 24℃；the two heat exposure groups were exposed to heat \[36℃\] 6 h daily). After 7 days of heat exposure, the body core temperature of rats was measured by telemetry technology. The response of isolated thoracic aorta to noradrenaline (NA) was determined by organ bath system. Expression of aorta eNOS mRNA and plasma NOx (NO2-and NO3-) content were also examined. Results Heat exposure showed no significant effects on body core temperature of rats (P>0.05). Compared with control group, the fixed-time heat exposure group had significantly lowered thoracic aortic vascular reactivity to NA (P<0.01). Compared with control group and random-time heat exposed group, the fixed-time heat exposure group had significantly increased eNOS mRNA expression and plasma NOx content (P<0.05).Conclusion Fixed-time exposure to heat can change the reactivity of the thoracic aorta in rats, and vascular endothelial cell-derived NOS pathway may be involved in the vasodilatory effects.