Low DA, Keller DM, Wingo JE, Brothers RM, Crandall CG. Sympathetic nerve activity and whole body heat stress in humans. J Appl Physiol 111: 1329-1334, 2011. First published August 25, 2011; doi:10.1152/japplphysiol.00498.2011.-We and others have shown that moderate passive whole body heating (i.e., increased internal temperature similar to 0.7 degrees C) increases muscle (MSNA) and skin sympathetic nerve activity (SSNA). It is unknown, however, if MSNA and/or SSNA continue to increase with more severe passive whole body heating or whether these responses plateau following moderate heating. The aim of this investigation was to test the hypothesis that MSNA and SSNA continue to increase from a moderate to a more severe heat stress. Thirteen subjects, dressed in a water-perfused suit, underwent at least one passive heat stress that increased internal temperature similar to 1.3 degrees C, while either MSNA (n = 8) or SSNA (n = 8) was continuously recorded. Heat stress significantly increased mean skin temperature (Delta similar to 5 degrees C, P < 0.001), internal temperature (Delta similar to 1.3 degrees C, P < 0.001), mean body temperature (Delta similar to 2.0 C, P < 0.001), heart rate (Delta similar to 40 beats/min, P < 0.001), and cutaneous vascular conductance [Delta similar to 1.1 arbitrary units (AU)/mmHg, P < 0.001]. Mean arterial blood pressure was well maintained (P = 0.52). Relative to baseline, MSNA increased midway through heat stress (Delta core temperature 0.63 +/- 0.01 degrees C) when expressed as burst frequency (26 +/- 14 to 45 +/- 16 bursts/min, P = 0.001), burst incidence (39 +/- 13 to 48 +/- 14 bursts/100 cardiac cyles, P = 0.03), or total activity (317 +/- 170 to 489 +/- 150 units/min, P = 0.02) and continued to increase until the end of heat stress (burst frequency: 61 +/- 15 bursts/min, P = 0.01; burst incidence: 56 +/- 11 bursts/100 cardiac cyles, P = 0.04; total activity: 648 +/- 158 units/min, P = 0.01) relative to the mid-heating stage. Similarly, SSNA (total activity) increased midway through the heat stress (normothermia; 1,486 +/- 472 to mid heat stress 6,467 +/- 5,256 units/min, P = 0.03) and continued to increase until the end of heat stress (11,217 +/- 6,684 units/min, P = 0.002 vs. mid-heat stress). These results indicate that both MSNA and SSNA continue to increase as internal temperature is elevated above previously reported values.