Atrial natriuretic factor (ANF), a family of peptides isolated from cardiac atria, has marked effects on sodium excretion. A synthetic 26 amino acid sequence of ANF peptide has also been shown to be phosphaturic. However, it is difficult to assess whether the phosphaturia is due to changes in tubular reabsorption of phosphate without control of filtered load of phosphate. In the present study, the hypothesis that ANF peptide decreases tubular phosphate reabsorption was tested by using graded phosphate infusions of 0, 1, 2, and 3 mumol/min in thyroparathyroidectomized rats. Further, reabsorbed phosphate was similarly assessed in rats infused with parathyroid hormone (PTH) to allow comparison with a known phosphaturic hormone. ANF peptide decreased reabsorbed phosphate compared with saline controls (2.72 +/- 0.28 mumol/ml GFR compared with 3.35 +/- 0.35, P less than 0.05) but not as much as a maximally phosphaturic dose of PTH (2.04 +/- 0.13 mumol/ml GFR). We conclude that synthetic ANF peptide decreases tubular phosphate reabsorption in vivo.