The effect of dietary iron overload on lipid peroxidation (LP), prostaglandin (PG) synthesis and lymphocyte proliferation was examined in young and old F344 rats. Rats 4 and 19-22 mo old were fed AIN-76 diet for 11-12 wk supplemented with 2.5% carbonyl iron obtained from two sources (Type A and B). Animals supplemented with Type A iron showed reduced food intake and weight gain associated with marked increases in extrahepatic and hepatic iron concentration. Rats receiving Type B iron had food intakes and body weights similar to those of controls but exhibited small increases in tissue iron concentration. Old control rats compared to young had significantly higher conjugated dienes (CD) in hepatic microsomes. Feeding Type A iron diets induced a significantly higher level of CD in hepatic microsomes from old rats compared to young rats. Iron overloaded rats also showed highly correlated (r = 0.94) increases in the urinary excretion of thiobarbituric acid-reactive substances and PG metabolites indicating increased in vivo LP and PG synthesis. Mitogen-stimulated PGE2 synthesis in young rats was increased at 4 wk in association with enhanced T-cell proliferation stimulated by Concanavalin A. Lymphocyte proliferation was significantly lower in old than in young control or iron-treated rats. The lack of efficacy of Type B vs. Type A iron appears due to a larger particle size and lower bioavailability. In conclusion, iron overloading increases in vivo LP and PG metabolism. Furthermore, the mitogenic response to Concanavalin A in young rats is enhanced after 4 wk of iron overloading.