Oxygen consumption (MO2) of confined eels was measured at atmospheric pressure, 1 atm, and at 101 atm of hydrostatic pressure per se (HP). The tolerance of the eels to hypoxia was studied at the two experimental pressures. At atmospheric pressure, when oxygen partial pressure (PWO2) fell below the critical pressure, Pc = 22.4 +/- 1.95 Torr, there was a linear PWO2-related decrease in MO2. At maximal hypoxia, the eels survived for several hours by their efficient anaerobic metabolism. At 101 atm of HP, as soon as the experimental pressure was attained, a linear PWO2-related decrease in MO2 was observed at PWO2 levels much higher than those considered as critical at atmospheric pressure. The relation MO2 = f (PWO2) was similar to that observed at 1 atm when PWO2 less than Pc, that is, when aerobic metabolism was insufficient to ensure the eels' energetic requirement. Moreover, the eels tolerated hypoxia much less well at 101 atm of HP than at 1 atm. In conclusion, the exposure of eels to 101 atm of HP induced a sharp decrease in aerobic metabolism; then, at 101 atm, the energetic requirements must be ensured by anaerobic processes which produced lactates in plasma whose values were similar to those observed at 1 atm when PWO2 less than Pc.