Heat stress evokes significant increases in muscle sympathetic nerve activity (MSNA) in healthy individuals. The MSNA response to heat stress in chronic heart failure (CHF) is unknown. We hypothesized that the MSNA response to heat stress is attenuated in CHF. Passive whole body heating was applied with water-perfused suits on 13 patients (61 ± 2 years) with stable class II-III CHF, 12 age-matched (62 ± 2 years) healthy subjects, and 14 young (24 ± 1 years) healthy subjects. Mild heating (i.e. increases in skin temperature Tsk ~2-4 °C, internal temperature Tcore <0.3 °C), significantly decreased MSNA in CHF patients, but did not significantly alter the MSNA in the age-matched and young healthy subjects. Heat stress (i.e. Tsk ~4 °C and Tcore ~0.6 °C) raised MSNA in the age-matched (32.9 ± 3.2 to 45.6 ± 4.2 bursts/min; P < 0.001) and young (14.3 ± 1.7 to 26.3 ± 2.4 bursts/min; P < 0.001) controls, but not in CHF (46.2 ± 5.3 to 50.5 ± 5.3 bursts/min; P = 0.06). The MSNA increase by the heat stress in CHF (4.2 ± 2.0 bursts/min) was significantly less than those seen in the age-matched (12.8 ± 1.7 bursts/min, P < 0.05) and young (12.0 ± 2.7 bursts/min, P < 0.05) control groups. These data suggest that the MSNA response to heat stress is attenuated in CHF patients. We speculate that the attenuated MSNA response to heat stress may contribute to impaired cardiovascular adjustments in CHF in a hot environment.