Three groups of rats were fed diets with either 10 weight percent (wt%) of evening primrose oil, safflower oil or soybean oil for 11 weeks. Diets contained 7.1 wt% linoleic acid + 0.8 wt% gamma-linolenic acid, 7.6 wt% linoleic acid, or 5.3 wt% linoleic acid + 0.7 wt% alpha-linolenic acid, respectively. In liver mitochondria as well as in heart, dietary gamma-linolenic acid did not affect the fatty acid profiles of phosphatidylcholines (PC), phosphatidylethanolamines (PE) or cardiolipins (CL), whereas dietary alpha-linolenic acid caused an increased formation of (n-3) polyunsaturated fatty acids (PUFA). The liver delta 6- and delta 5-desaturase activities determined in vitro were not affected by the dietary fats. In brain PE, which are rich in C22- and C20-(n-3) PUFA, as well as in testes PC and PE, which are rich in (n-6) PUFA, no effects were found from a partial replacement of dietary linoleic acid with gamma-linolenic acid or alpha-linolenic acid. In kidney PC, PE, phosphatidylinositol (PI) and CL, 20:3(n-6) was moderately elevated to ca. 1% following intake of gamma-linolenic acid, whereas partial replacement of linoleic acid with alpha-linolenic acid was followed by increased deposition of 22:6(n-3) in PC and PE of testes and kidney. Thus, no general effect of evening primrose oil on the content of (n-6) PUFA in rat tissue phospholipids was observed, whereas a significant incorporation of gamma-linolenic acid into liver and adipose tissue triglycerides was found.