Effect of whole body gamma irradiation (1200 R) on the fatty acid composition of liver lipids and its triglycerides (TG), phosphatidyl ethanolamine (PE) and phosphatidyl choline (PC) has been studied in female rats. Radioprotective effect of cystamine on radiation-induced alterations in fatty acid composition of above liver lipid fractions has been studied by giving crystamine 15 min before irradiation. Irradiation increased palmitic acid levels in liver total lipids, and PE and decreased in TG. Cystamine prevented these changes. Irradiation increased palmitoleic acid levels in liver total lipids, total phospholipids and PC and these were prevented by prior administration of cystamine. Linoleic acid was decreased in liver total lipids, TG, total phospholipids and increased in PE and PC of irradiated rats. Administration of cystamine before irradiation partially protected these changes. Arachidonic acid was reduced in all liver lipid fractions of irradiated rats and this was only partially protected by cystamine, which itself reduced its levels in the control animals. Irradiation increased the levels of eicosatrienoic acid and these were not prevented by cystamine.