Organic phosphates and the Donnan ratio (cell to plasma Cl ratio) were measured in man during a dry saturation dive to 31 ATA for 7 d. 2,3-DPG increased significantly with a concomitant decrease in ATP and the molar ratios of ATP and ADP on Day 5 at 31 ATA (P less than 0.05). Inorganic phosphate did not change during the dive. The Donnan ratio was relatively high at 31 ATA and the value was significantly different on Day 3 at 31 ATA (0.731 at 1 ATA vs. 0.895 at 31 ATA, P less than 0.05). This suggests a rise in intracellular pH, resulting from a change in the net charge of hemoglobin. It is speculated that an increase in oxygen affinity of hemoglobin occurs during the early phase at 31 ATA and is followed by an increase in 2,3-DPG in response to the resulting tissue hypoxia.