Serum and liver tissue fatty acid and lipid fraction values were studied in rats kept for various periods on a hyperlipid, hypoprotein, steatogenous diet, with or witout choline. There was a general increase in liver fats due to an absolute increase in triglycerides, accompanied by changes in the fatty acid methyl ester composition, indicating the storage of lipids of lesser metabolic activity and more similar to those of peripheral fat (palmitic and oleic acid increments; decreases in myristic, stearic, linoleic and arachidonic acid values). These changes were partly reflected in the blood picture: little difference in lipid fraction ratios, coupled with fatty acid gas chromatograms similar to those observed for the liver. The omission of choline aggravated the alterations caused by the experimental diet.