The effects of maleic acid on renal phosphate (P(i)) transport were examined by clearance and brush-border membrane vesicle (BBMV) transport studies. In normal rats, maleic acid 50 mg·kg body wt-1·h-1 increased the phosphaturia (P < 0.001). Intraperitoneal administration of a similar dose of maleic acid decreased the BBMV uptake of P(i) but not glucose. In rats fed a low-phosphate diet (0.03%), the maleic acid-induced phosphaturia was blunted, but the inhibitory effect on the BBMV transport of P(i) persisted. In chronic parathyroidectomized rats fed a low-phosphate diet, where the filtered load of P(i) was higher than in the previous groups, the phosphaturia was abolished, but the inhibition of the BBMV transport of P(i) was sustained. Both the in vitro incubation of BBMVV and in vivo administration of maleic acid were associated with a competitive inhibition of P(i) transport. These studies indicate that the maleic acid-induced phosphaturia is expressed at the apical membrane entry step of P(i), and the enhanced distal tubular reabsorption accounts for the lack of phosphaturia in dietary P(i) deprivation.