The purpose of this study was to determine if the arterial baroreceptor reflexes modulate the sympathocirculatory responses to acute heat stress. To address this, arterial pressure, heart rate, mesenteric and renal blood flow velocity (Doppler flow probes), arterial plasma norepinephrine, and colonic temperature were measured before and during whole body heating (42° C ambient temperature) in groups of conscious, unrestrained rats with (sham) or without (sinoaortic deafferentation) intact arterial baroreceptor reflexes. Heating was stopped when a colonic temperature of 41° C was attained. Baseline levels of arterial pressure were similar in the two groups, whereas heart rate was elevated in deafferented versus sham-operated rats (p<0.01). The increases above baseline for both arterial pressure (73±4 vs. 27±2 mm Hg) and heart rate (127±10 vs. 33±5 beats/min) were threefold to fourfold greater at the end of heating in the deafferented versus the sham group (/7<0.01). Declines in mesenteric and renal blood flow were similar in the two groups during heating; however, deafferented rats had greater increases in both mesenteric and renal vascular resistance (p<0.05). Plasma norepinephrine was elevated at baseline in deafferented versus sham rats and increased in both groups during heating (/?<0.01). The magnitude of the increase in plasma norepinephrine from baseline to 41° C was fivefold greater in the deafferented versus the sham rats (p<0.01). Furthermore, deafferented rats reached a colonic temperature of 41° C much faster than the sham rats (38±6 vs. 94±13 minutes), resulting in a threefold greater heating rate (p<0.01). These findings indicate that the arterial baroreceptors modulate the arterial pressure, heart rate, and visceral vascular resistance responses to nonexertional heat stress in the conscious rat and suggest that this modulation is mediated, at least in part, via baroreceptor inhibition of central sympathetic outflow. Moreover, thermal tolerance during prolonged heat exposure is in part dependent on intact arterial baroreceptor reflexes.