The relationship between membrane function and energy metabolism was studied in rainbow trout hepatocytes, an anoxia-intolerant cell system, and compared with the situation in hepatocytes from the goldfish, a typical anoxia-tolerant species. In trout hepatocytes, under normoxia and under chemical anoxia, inhibition of ATP consumption by the Na+ pump induced a decrease in ATP production of the same magnitude. In response to chemical anoxia, total ATP production was reduced to 15% and Na+ pump activity to 22% of the control rate under normoxia. Measurement of the cellular ATP content under these conditions revealed that, despite the reduction in Na+ pump activity, the cells became rapidly depleted of ATP, with the time course of this process resembling that observed in the anoxic rat hepatocyte. This is in contrast to the responses of goldfish hepatocytes, where, during chemical anoxia, 1) inhibition of the Na+ pump did not lead to a corresponding reduction in ATP production and 2) ATP levels, after a transient decrease, stabilized at a new steady state. To investigate the consequences of chemical anoxia on ion homeostasis, efflux and uptake rates of K+ were determined simultaneously. In the trout cells, chemical anoxia led to a decoupling of influx and efflux rates, the latter exceeding the former three- to eightfold. In contrast, goldfish hepatocytes were able to preserve ion homeostasis by a concerted decrease in Rb+ uptake and K+ efflux, so that the net flux of K+ was always close to zero. In neither species did chemical anoxia induce a change in pump density. Other potential control mechanisms are briefly discussed.