Gill ventilation, stroke volume and frequency, %O2 utilization and oxygen uptake, and dorsal aortic blood oxygen tension, content, pH and oxygen affinity have been determined during normoxia and during a range of hypoxic exposures in the sturgeon, Acipenser Transmontanus. In air-equilibrated water gill ventilation was 350 ml/kg/min, % utilization was 35--40%, and oxygen uptake at 15 degrees C was 55--60 ml O2/kg/h. Dorsal aortic blood PO2 was 90 mm Hg and blood O2 content at a normal pHa of 7.84 was 7.0 vol%. Vg fell considerably through a reduction in branchial stroke volume when PIO2 was reduced from 150 to 100 mm Hg. Although % utilization remained unchanged, VO2 was halved, clearly identifying Acipenser as an O2 conformer with a critical O2 tension just below air saturation. At a PIO2 of 60 mm Hg VO2 was only 15% of that at normoxic levels falling to only 5% at a PIO2 of 30 mm Hg. There was no hypoxic bradycardia. There was no repayment of an oxygen debt even after severe hypoxic exposure in Acipenser, and pHa remained unchanged under all experimental conditions, a response incompatible with lactate or succinate production. It is concluded that the sturgeon reduces total energy expenditure during hypoxic exposure, rather than switching from aerobic to anaerobic metabolism.