To determine if drinking fluid altered exercise and thermal variables during 2-h immersions in 25 degrees C water, 11 male subjects were tested breathing air at 1 ATA and HeO2 at 5.5 ATA (PO2 = 0.42 ATA). Each immersion consisted of four periods of 5-min rest, 5-min leg exercise at 50 W, and 20 min of exercise at 68 +/- 3% VO2max. One test at each depth was done with no fluid (NF); one test was done with subjects consuming 125 ml of a 7% glucose polymer (GP) solution every 30 min (total intake = 500 ml). Compared with NF, GP increased urine volume by 469 +/- 118 and 443 +/- 82 ml at 1 and 5.5 ATA. Mean minute ventilation (VE) was reduced at depth by 10.1 +/- 0.6 and 5.1 +/- 0.6 l.min-1 for NF and GP, respectively (P < 0.05). GP trials reduced ventilatory equivalent (VEQ, VE/VO2) at 1 ATA, but increased it at 5.5 ATA, secondary to a small change in VE. Exercise heart rate was slightly lower at 5.5 ATA, but O2 pulse was the same among all conditions (17.2 +/- 0.1 ml O2.beat-1). Rectal temperature increased 0.71 +/- 0.04 degrees C for all conditions. Total body heat flux and insulation were unaffected by fluid or hyperbaric treatments. These results indicate that drinking fluid during immersed exercise does not change fluid balance or thermal status at depths to 5.5 ATA. The modest changes in VEQ noted with fluid intake or hyperbaric exposure reflect inconsequential alterations in cardiopulmonary efficiency.