Physiological dead space (Vds), end-tidal CO(2) (Pet(CO(2))), and arterial CO(2) (Pa(CO(2))) were measured at 1 and 2.8 ATA in a dry hyperbaric chamber in 10 older (58-74 yr) and 10 younger (19-39 yr) air-breathing subjects during rest and two levels of upright exercise on a cycle ergometer. At pressure, Vd (liters btps) increased from 0.34 +/- 0.09 (mean +/- SD of all subjects for normally distributed data, median +/- interquartile range otherwise) to 0.40 +/- 0.09 (P = 0.0060) at rest, 0.35 +/- 0.13 to 0.45 +/- 0.11 (P = 0.0003) during light exercise, and 0.38 +/- 0.17 to 0.45 +/- 0.13 (P = 0.0497) during heavier exercise. During these conditions, Pa(CO(2)) (Torr) increased from 33.8 +/- 4.2 to 35.7 +/- 4.4 (P = 0.0059), 35.3 +/- 3.2 to 39.4 +/- 3.1 (P < 0.0001), and 29.6 +/- 5.6 to 37.4 +/- 6.5 (P < 0.0001), respectively. During exercise, Pet(CO(2)) overestimated Pa(CO(2)), although the absolute difference was less at pressure. Capnography poorly estimated Pa(CO(2)) during exercise at 1 and 2.8 ATA because of wide variability. Older subjects had higher Vd at 1 ATA but similar changes in Vd, Pa(CO(2)), and Pet(CO(2)) at pressure. These results are consistent with an effect of increased gas density.