Pre- and postbranchial blood was sampled from crabs, Carcinus maenas, in control normoxic conditions and after 3, 24 and 72 hours exposure to hypoxic water (PWO2 = 30 Torr) at 15 degrees C. Oxygen partial pressures, pH and the concentrations of L-lactate, urate and hemocyanin were measured and an in vitro relationship between P50 and pH was determined on a pooled sample for each series, allowing an estimation of the average in vivo oxygen affinity. In separate experiments, in vitro additions of urate was found to increase hemocyanin oxygen affinity in dialyzed hemolymph. In normoxia acclimated animals, high PaO2 values (107 +/- 8 Torr) prevailed and oxygen transport relied mainly on physically dissolved oxygen (80%). During hypoxic exposure, the fall of both PaO2 (down to 8-17 Torr) and PvO2 (4-8 Torr) was accompanied by an increase in in vivo hemocyanin oxygen affinity (delta P50 = -3 to -6 Torr, compared to normoxic blood). This increase was almost fully explained by the concomitant action of elevated blood pH, urate concentration and, to a lesser extent, L-lactate concentration. As a result, the difference between pre- and postbranchial oxygen concentration was maintained, or even increased, during hypoxia, the major part of the oxygen being transported as oxyhemocyanin (94-98%). We conclude that pH and urate are the most effective modulators in enhancing oxygen binding to hemocyanin during moderate ambient hypoxia. Whether an increase in hemocyanin oxygen affinity is essential to hypoxia adaptation in crustaceans is discussed.