The composition of the fatty acids in the thymus, spleen and mesenteric lymph nodes was determined in rats fed polyunsaturated (UFC) or saturated (SFC) fatty acid-rich chow during 6 weeks or 14 months. The results indicated that the lipid composition of fatty acids in these tissues was modified by the type of fat given in the diets. Interestingly, the liver did not show any dietary induced change in the composition of fatty acids. The unsaturation index was raised in the lymphoid organs by UFC either after 6 weeks or 14 months. The ageing process itself increased the degree of unsaturation of fatty acids only in the spleen of the 3 groups. A high degree of unsaturation of fatty acids in the tissues may favour the occurrence of lipid peroxidation. It was noteworthy that a linoleic acid-rich diet (UFC) did not change the content of arachidonic acid in the tissues and so would therefore be unlikely to affect eicosanoid synthesis. As shown by previous studies, these fat-rich diets caused marked changes in the key enzyme activities of glucose and glutamine metabolism in the lymphoid organs, by as yet unknown mechanisms. The results reported here suggest that the effect of fat-rich diets on intermediary metabolism does not occur through eicosanoid synthesis and may be a consequence of the lipid peroxidative process or even alterations in the transcription of the enzymes of glycolysis and glutaminolysis.