We present evidence that oxygen consumption (VO2) is oxygen partial pressure (PO2)dependent in striated muscles and PO2-independent in the vasculature in representatives of three craniate taxa: two teleost fish, a hagfish and a rat. Blood vessel VO2 displayed varying degrees of independence in a PO2 range of 15-95 mmHg, while VO2 by striated muscle tissue slices from all species related linearly to PO2 between 0 and 125 mmHg, despite VO2 rates varying greatly between species and muscle type. In salmon red muscle, lactate concentrations fell in slices incubated at a PO2 of either 30 or 100 mmHg, suggesting aerobic rather than anaerobic metabolism. Consistent with this finding, potential energy, a proxy of ATP turnover, was PO2-dependent. Our data suggest that the reduction in VO2 with falling PO2 results in a decrease in ATP demand, suggesting that the hypoxic signal is sensed and cellular changes effected. Viability and diffusion limitation of the preparations were investigated using salmon cardiac and skeletal muscles. Following the initial PO2 depletion, reoxygenation of the Ringer bathing salmon cardiac muscle resulted in VO2S that was unchanged from the first run. VO2 increased in all muscles uncoupled with p-trifluoromethoxylphenyl-hydrazone (FCCP) and 2,4-dinitrophenol (DNP). Mitochondrial succinate dehydrogenase activity, quantified by reduction of 3-(4,5-dimethylthiazol)-2,5-diphenyl-2H-tetrazolium bromide (MTT) to formazan, was constant over the course of the experiment. These three findings indicate that the tissues remained viable over time and ruled out diffusion-limitation as a constraint on VO2.