The effect of dietary conjugated linoleic acid (CLA) isomers (CLAmix), Na₂SeO₃ (SeIV) or selenized yeast (SeY) on spleen weight, contents of CLA isomers and other fatty acids (FA), and efficiency of fatty acid desaturation in the spleen and blood plasma of rats were investigated. The study was performed on 80 female Wistar rats, 8 weeks of age with an initial body weight of 195.4±0.8 g. Each group numbered 8 rats. For 6 weeks the rats were fed ad libitum the Labofeed H diet supplemented with 1.5% CLAmix, 0.2 ppm Se as sodium selenite (_LSeIV), selenized yeast (_LSeY), or 0.5 ppm Se as SeIV (_HSeIV) or SeY (_HSeY). The addition of CLAmix to the diet with _LSeIV or _HSeIV increased the weight of the spleen in comparison with rats fed the diet with SeIV or the control group. The diets enriched in CLAmix, _HSeY, or the diet with CLAmix and SeY (as _LSeY or _HSeY) decreased spleen weight. The diet containing CLAmix, with or without Se, as SeIV or SeY, reduced the sum of fatty acids (ΣFA) in the spleen, while increasing it in the plasma. The diet with CLAmix and _HSeY most efficiently increased the content of CLA isomers, especially c9t11CLA, in the spleen and plasma. C9t11CLA in the spleen was metabolized more efficiently than t10c12CLA, while in plasma t10c12CLA was metabolized faster than c9t11CLA. The diet containing CLAmix with or without SeIV or SeY, significantly decreased the contents of c9C18:1, c11C18:1, and sums of saturated fatty acids (SFA), mono-(MUFA) and polyunsaturated fatty acids (PUFA) in the spleen, whereas the diet containing CLAmix and/or _HSeY increased the concentrations of these fatty acids in plasma. The diet enriched in CLAmix or _HSeY decreased the ratios of PUFA/SFA, MUFA/SFA and PUFA/ΣFA in the spleen and plasma. The addition of SeIV or SeY to the diet with CLAmix usually slightly decreased these ratios in the spleen and plasma in comparison with the diet containing CLAmix. Our results indicate that the diet with CLAmix more significantly stimulated the metabolism of PUFA and MUFA in the spleen than of SFA. The addition of SeIV or SeY to the diet with CLAmix increased the metabolic efficiency of PUFA and MUFA in the spleen.