Methyl linoleate (18:2 omega 6) was administered to rats deficient in essential fatty acids (EFA) and the effects on osmotic fragility and lipid composition of their red blood cells were studied. Even if they remained deficient in omega-3 fatty acids, the fragile red cells of EFA-deficient rats were remarkably strengthened by the linoleate treatment. It is concluded that omega-3 fatty acids are not essential for the maintenance of cell membrane integrity in rats. Since the cholesterol/phospholipid ratio in red cells was lower in untreated than in treated rats, it is suggested that this factor might be responsible in part for the higher fragility of red cells in EFA-deficient rats. Despite the prolonged administration of overload quantities of methyl linoleate, the proportion of 18:2 omega 6 in the red blood cell phospholipids of treated rats was actually lower than that in rats fed a regular stock diet, while the relative content of other omega-6 fatty acids (20:4 omega 6, 22:4 omega 6, and 22:5 omega 6) increased. These results indicate that the metabolic processes of omega-6 fatty acids in treated rats were stimulated, probably as a result of the lack of competition from omega-3 fatty acids for the same enzyme systems.