(1) The use of an extracorporeal blood circulation method has made it possible to obtain information on the time course of possible modifications in acid-base status of trout blood during hypoxia. (2) Two series of experiments are described: (a) Continuous measurement of arterial pH, PCO2 and PO2 during 24 h of moderate hypoxia (60 Torr). (b) Continuous measurement of acid-base status during 24 h of deep hypoxia (40 Torr). (3) The results of the first series show that hypoxia provokes a stimulation of ventilatory frequency and amplitude which is associated with an increased output of CO2 from the blood which induces respiratory alkalosis. This alkalosis is compensated during the following 24 h, and results in a return to the initial pH, a decrease in concentration of HCO-3 + CO2-(3) and an increase in lactate levels and Cl- ion concentration. (4) The second set of experiments show that in deep hypoxia the respiratory alkalosis is preceded by a brief stage of deep metabolic acidosis during which lactate levels suddenly increase dramatically.