Previous studies in rats have demonstrated that 1) aldosterone enhances biosynthesis of renal flavin coenzymes; 2) riboflavin analogs inhibit the synthesis of aldosterone; and 3) adriamycin inhibits flavin coenzyme biosynthesis. In their entirety, these findings suggest that both diminished flavin coenzyme biosynthesis induced by adriamycin and a dietary riboflavin deficiency would result in decreased formation of aldosterone. The present study examined the effects of adriamycin treatment on serum aldosterone in rats consuming either a diet adequate in riboflavin or a riboflavin-deficient diet. Groups of rats fed specially prepared diets were injected for 3 days with adriamycin (cumulative dose range, 6-24 mg/kg BW). Pair-fed controls were given saline. After death, adrenal glands were excised, and blood samples were analyzed for aldosterone levels. No changes in adrenal weights or protein and potassium concentrations were observed after adriamycin treatment. In contrast to initial predictions, in riboflavin-sufficient rats, serum aldosterone levels were markedly enhanced by adriamycin in a doserelated manner. Riboflavin-deficient animals had lower basal aldosterone levels and markedly attenuated responses to adriamycin than did riboflavin-sufficient rats. In separate groups of adriamycin-treated rats fed a normal chow diet, serum aldosterone levels increased, and serum corticosterone levels showed a small but significant decline. In addition, adriamycin treatment caused an increase in urinary potassium excretion and a decrease in sodium excretion. These results suggest that flavins play a decisive role in regulating the levels of aldosterone and raise the possibility that the adriamycin-induced increase in serum aldosterone may be part of the pathogenetic mechanisms of cardiovascular toxicity and overall muscular weakness.