Perfusion of livers with whole blood containing carboxyhaemoglobin decreased hepatic O(2) consumption and triglyceride secretion and raised free fatty acid oxidation. 2. Perfusion with [(14)C]carboxyhaemoglobin indicated that there was negligible hepatic uptake of (14)CO. 3. The observations appear to be due to a decrease in O(2) consumption rather than to specific effects of carboxyhaemoglobin.