We have studied the mechanisms of acute hypoxia tolerance in rainbow trout (Salmo gairdneri). Fish held at 9 degrees C were exposed to various levels of hypoxia for 24 h. At an environmental PO2 of 30 Torr, the fish showed an initial plasma acidosis probably of metabolic origin which was subsequently offset such that blood pH returned to normal within about 4 h. Over this time period, red cell pH was maintained constant. Comparing the effects of different levels of hypoxia following 24 h exposure, oxygen consumption of the animal remained unchanged over a broad range of inspired oxygen tensions but declined by over 30% of normoxic values at inspired water PO2 levels of 80 Torr. This appeared to be a true metabolic depression because signs of increased anaerobic metabolism did not occur until there was a further reduction in water oxygen levels. Rainbow trout appear to be able to maintain a relatively high energy status in their white muscle during 24 h exposure to severe hypoxia (water PO2 = 30 Torr). As the level of hypoxia was intensified, there was a reduction in the oxygen gradient across the gills, probably facilitated in part by the release of catecholamines into the blood. The erythrocytic ATP: Hb4 molar ratios declined with increasing hypoxic stress as did the pH gradient between the erythrocyte and plasma. The overall effect was no change in Hb O2-affinity after 24 h exposure to severe hypoxia.