Publisher Summary This chapter examines the red blood cell metabolism in fishes. The potential for the activity of several pathways in fish blood has been demonstrated in several studies. It presents a study of whole blood and washed erythrocytes of the sea raven, Hernitripterus arnericanus, in which the rates of glucose utilization and O2 consumption are compared. It was found that in whole blood, the calculated O2 consumption rates from the measured disappearance of total glucose very closely matched measured O2 consumption rates. The match of respiration rate and glucose consumption was less precise in washed erythrocytes and O2 consumption rates of washed, unstimulated erythrocytes were approximately 50% of those for whole blood, but glucose consumption remained the same, even when O2 consumption was doubled by maximal adrenergic stimulation. It was observed that rainbow trout RBCs oxidized lactate poorly, but exhaustive exercise increased lactate oxidation to exceed that of glucose. The increase in lactate oxidation rate is far above that attributable to the increase in substrate concentration alone. This implies that a catecholamine-induced process may be involved.