The present study was designed to evaluate the influence of dietary nucleotides on the lipid composition of liver microsomes in weanling rats. Rats at weaning were fed, one group with a semipurified nucleotide-free diet and three groups with the same diet supplemented with three different levels of each of the nucleotides AMP, GMP, IMP, UMP and CMP, during 4 weeks. Long chain polyunsaturated fatty acids of the n-6 and n-3 series were increased in phospholipids of liver microsomes for rats fed the nucleotide sypplemented diets; however, the cholesterol/phospholipid phosphorus ratio was maintained fairly constant. The results obtained suggest that dietary nucleotides modify the polyunsaturated fatty acid metabolism through a lower rat liver delta-9 desaturase activity and through increased activities of delta-5 and delta-4