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The effects of two non-protein toxic amino acide , indospicine and canavanine, and super oxide radicals was tested using cultured rat peritoneal macrophages. Both compounds were able to increase superoxide production induced by phorbol myristate acetate (PMA) in rat peritoneal macrophages. The amount of super oxide was measured spectrophotometrically by reduction of cytochrome C in the absence and presence of Super oxide dismutase (SOD). It was found that the increase in super oxide production was due to decreased nitric oxide (NO) synthesis. The synthesis of No in cultured macrophage was mediated by an inducable NO synthase. It is concluded that the toxicity of indospicine and canavanine may exerted through an increase in super oxide production, and the increase in super oxide is mediated by the inhibiton of NO by indospicine and canavanine as L- arginine analogues.