Superoxide anion and nitric oxide production have been studied in resident and activated peritoneal macrophages of 3-, 12-, and 24-month-old rats. Some key enzymes involved in the metabolism of glucose were also studied in relation to aging. Production of O2 and NO was reduced in all cases in middle-aged (12 months) and old (24 months) animals. Malic enzyme and citrate synthase activity shows a progressive reduction with age. Hexoquinase, pyruvate quinase, and lactate dehydrogenase activities decrease sharply from 3 to 12 months with no significant change between 12 and 24 months. Taken as a whole, the results of enzyme activity suggest that aging may reduce the capacity for glucose utilization in macrophages.