This project will investigate the role of naturally increased endogenous carbon monoxide (CO) in a deep-diving marine mammal. Several species of deep-diving seals have concentrations of carbon monoxide (CO) in their blood that resemble the levels seen in chronic cigarette smokers. The proposed work measures the turnover of red blood cells in marine mammals as a potential source of CO production and examines the effects of elevated CO on the oxygen binding characteristics of blood and the regulation of mitochondrial metabolism during breath-holds.