Maleate administration produces features that closely resemble the Fanconi syndrome. To determine whether this dysfunction is caused by maleate or its metabolite, maleyl-CoA, which is produced in the succinyl-CoA transferase reaction, the effect of pretreatment with acetoacetate on the maleate dysfunction was tested in rats, Infusion of acetoacetate, 90 mu mol . min-1 . kg body wt-1 protects the kidney from maleate action, whereas administration of propionate, a monocarboxylic anion and CoA-dependent metabolite resembling the anion of acetoacetate, has no effect on maleate-induced renal dysfunction. Maleate (100 mg/kg body wt) alone lowered kidney ATP concentration by 44%, whereas maleate with acetoacetate did not prevent the decreased renal ATP (42%), while glucosuria, phosphaturia, calciuria, and bicarbonaturia were significantly diminished. Similar changes in renal ATP level were observed in rats treated with propionate or propionate and maleate in combination. These experiments demonstrate that maleate per se is not inhibitory but that its metabolite, presumably maleyl-CoA, may inhibit tubule function.