Aging is associated with decline in the functioning of immune cells and reductions in serum L-arginine and excretion of nitric oxide metabolites. Studies have shown that L-arginine plays an important role in many physiological, biological and immunological processes. The present study was performed to determine if treatment with L-arginine could prevent age-related changes in phagocytic function of peritoneal macrophages. The effects of L-arginine on phagocytic activity of peritoneal macrophages were compared between young and middle-aged rats. Studies were performed in four groups of rats for 8 weeks: group 1 (3 month-old) received physiological saline; group 2 (3 month-old) received L-arginine (160 mg/kg/day); group 3 (12 month-old) received physiological saline; group 4 (12 month-old) received L-arginine (160 mg/kg/day). There were no significant differences in percentage of cells which were phagocytized. However, the phagocytosis of activated charcoal by peritoneal macrophages reduced with age. Thus, the phagocytic index was lower in macrophages of middle-aged rats. L-arginine treatment increased phagocytosis by peritoneal macrophages of both young and middle-aged rats. L-arginine-induced augmentation in phagocytosis by macrophages were much higher in the middle-aged rats compared with young rats. In summary, we found that L-arginine prevented the age-related reduction in phagocytic capability of peritoneal macrophages.