The effects of biotin and protein-calorie deficiency on metabolism of liver phospholipids in rats were determined. Weanling male Long-Evans rats were fed ad libitum a biotin-deficient (0.4 micrograms biotin/g diet) or a biotin-adequate (2.4 micrograms biotin/g diet) diet for 49 days. A pair-fed group was fed biotin-adequate diet but restricted to the amount consumed by biotin-deficient rats. The effects of these diets on food intake, weight gain, biotin content of serum and fatty acid composition of liver phospholipid were determined. Growth and food intake were significantly depressed in biotin-deficient and pair-fed rats. Biotin concentration in serum was significantly reduced in biotin-deficient and in pair-fed rats. Biotin deficiency caused significantly increased concentrations of 15:0, 17:0 and total odd-chain fatty acids in liver phospholipids. Combined biotin and protein-calorie deficiency and protein-calorie deficiency alone caused significantly increased concentrations of 18:2 omega 6 in liver phospholipids. Biotin deficiency caused significant reduction in liver phospholipid 20:3 omega 6. Reduced arachidonic acid (20:4 omega 6) level was due to combined biotin protein-calorie deficiency. Protein-calorie deficiency alone was primarily responsible for reduced levels of 22:5 omega 6. Biotin deficiency alone caused significant increases in liver phospholipids 18:3 omega 3 and 22:6 omega 3.