Ä6-Desaturation of linoleic acid and Ä5-desaturation of dihomo--linolenic acid were measured in liver microsomes from ra fresh Baobab seed oil containing cyclopropene fatty acids (malvalic acid and sterculic acid) or heated Baobab seed oil practically devoid of these fatty acids or control oil. The presence of cyclopropene fatty acids in the fresh Baobab oil diet highly depressed both desaturations, but Ä6- more than Ä5-desaturation. The decreased capacity of microsomes to desaturate was reflected in the lower arachidonic acid content in microsomal phospholipids from rats fed this oil. However it was also lower in rats fed heated oil although in vitro Ä6- and Ä5-desaturation were not depressed. When liver microsomes prepared from rats fed the control diet were used for the desaturation assays, the presence of free malvalic or sterculic acid in the medium, also highly depressed Ä6- and Ä5-desaturation. The incorporation of arachidonic acid, the product of Ä5-desaturation, into phospholipids was also highly depressed, while that of the precursor dihomo--linolenic acid was not. This suggests that cyclopropene fatty acids specifically inhibit incorporation of the Ä5-desaturation product into phospholipids or that they specifically inhibit desaturation of the substrate previously incorporated into a membrane phospholipid. 