Intravenous (i.v.) infusion of excessive energy has been associated with hepatic steatosis. The time course of liver lipid accumulation was examined during 6 days of i.v. hyperalimentation with fat-free infusate. Adult male rats with indwelling superior vena cava cannulas received a dextrose-amino acid infusate for 0, 1/2, 1, 2, 4 or 6 days to provide 146% of nonprotein energy requirement [congruent to 350 non-protein kcal/(kg . day)] and 335% of nitrogen requirement [congruent to 2.7 g amino nitrogen/(kg . day)]. Significant hepatomegaly was apparent by day 1/2. Initially, glycogen deposition accounted for the liver enlargement, but after day 2, liver glycogen was declining and liver lipid was increasing. By day 4, liver lipid had increased fourfold and was the major contributor to hepatomegaly. Concurrent with fatty liver metamorphosis, hepatic essential fatty acid deficiency (EFAD) developed by day 4; liver linoleic acid levels had dropped from 20 to 1% of total fatty acids, and liver triene:tetraene ratio was 0.68. Similar changes in hepatic phospholipid fatty acids were observed. Enhanced lipogenesis and impaired lipid transport is known to accompany EFAD and may underlie the observed steatosis. A doubling of plasma cholesterol levels was also associated with steatosis. The mechanism leading to this increase in plasma cholesterol warrants further investigation.