The minimum arterial O2 partial pressure (PaO2) at which, in resting conditions, O2 consumption (MO2) can be maintained and below which anaerobic metabolism is initiated was studied in the crabs Eriocheir sinensis and Carcinus maenas at 15 degrees C. Arterial PO2, MO2 (in E. sinensis), blood lactate concentration ([lact]b) and blood copper concentration ([Cu]b, an index of the blood O2 carrying capacity) were determined after 24 h exposure to inspired PO2 (PIO2) ranging from 2.7-2.1 kPa. They were compared to normoxic controls. In normoxia, the most frequently measured PaO2 ranged between 1 and 3 kPa in both species. In hypoxia, the threshold for blood lactate appearance was PaO2 = 2.1 kPa in E. sinensis and 1.3 kPa in C. maenas, but in many individuals anaerobic metabolism was initiated at lower PaO2's. The lowest PaO2 with [lact]b approx. 0 was 0.7 kPa in both species. MO2 was maintained in 4 E. sinensis out of 6 with PaO2 ranging from 0.7-1.2 kPa (PIO2 = 2.1 kPa). The arterial PO2 at which anaerobic metabolism occurred was not related to blood O2 carrying capacity.