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The effect of dietary intake of specific types of fatty acids on retinal degeneration due to N-methyl-N-nitrosourea (MNU)-induced photoreceptor cell apoptosis was evaluated. Fifty-day-old female Sprague-Dawley rats were given a single intraperitoneal injection of 50 mg kg(-1) body weight of MNU, and were then switched to one of five different diets containing the following fatty acids at the following weight percentages: 10% linoleic acid (LA); 9.5% palmitic acid (PA) and 0.5% LA; 9.5% eicosapentaenoic acid (EPA) and 0.5% LA; 4.75% EPA, 4.75% docosahexaenoic acid (DHA) and 0.5% LA; or 9.5% DHA and 0.5% LA. When rats developed MNU-induced mammary tumors with a diameter of > or =1 cm, or at the termination of the experiment (20 weeks after MNU injection), retinal tissue samples were obtained and examined. Incidence and severity of retinal damage were compared by histologic examination. MNU-induced retinal degeneration was prevented in rats fed the diet containing 9.5% DHA (4.75% DHA was less effective), whereas it was accelerated in rats fed the 10% LA diet. Over the course of the 20-week experimental period, the fatty acid composition of serum reflected differences in dietary fatty acids. The present results indicate that a diet containing 9.5% DHA can counteract MNU retinotoxicity in the rat retina. DHA may play a role in protection against MNU-induced photoreceptor cell apoptosis in the rat retina.