We sought to determine whether hydrostatic pressure contributed to bradycardia and hypoventilation in hyperbaria. Eight men were studied during exercise at 50, 150, and 250 W while breathing 1) air at 1 bar, 2) helium-oxygen (He-O₂) at 5.5 bar, 3) sulfur hexafluoride-oxygen (SF₆-O₂) at 1.3 bar, and 4) nitrogen-oxygen (N₂-O₂) at 5.5 bar. Gas densities were pairwise identical in 1) and 2), and 3) and 4), respectively. Increased hydrostatic pressure to 5.5 bar resulted in a modest but significant relative bradycardia on the order of 6 beats/min, in both the absence [1) vs. 2), P = 0.0015] and presence [3) vs. 4), P = 0.029] of gases that are both denser than normal and mildly narcotic. In contrast, ventilatory responses appeared not to be influenced by hydrostatic pressure. Also, the combined exposure to increased gas density and mild-to-moderate inert gas narcosis at a given hydrostatic pressure [1) vs. 3), 2) vs. 4)] caused bradycardia (P = 0.032 and 0.061, respectively) of similar magnitude as 5.5-bar hydrostatic pressure. At the same time there was relative hypoventilation at the two higher workloads. We conclude that heart rate control, but not ventilatory control, is sensitive to relatively small increases in hydrostatic pressure.