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The effect of NaHCO₃ infusion on phosphate reabsorption by the proximal convoluted tubule was studied in acute and chronic PTX rats. Either hypertonic NaHCO₃ or NaCl was infused intravenously. Proximal fluid and phosphate reabsorption were measured with ³H-inulin and ³²PO₄, respectively. In acute PTX rats, the fraction of filtered ³²PO₄ remaining in the proximal lumen was 5.9% during NaCl infusion and 23% during NaHCO₃ infusion (p < 0.001). In the chronic PTX rats, the fraction of filtered ³²PO₄ remaining in the proximal lumen was 24% during NaCl infusion and 38% during NaHCO₂ infusion (p < 0.001). Phosphate reabsorption per unit GFR was reduced by NaHCO₂infusion in both acute and chronic PTX rats. A linear relationship was found between tubular fluid total CO₂ and the fraction of ³²PO₄ remaining in the tubular lumen. The findings indicate that NaHCO₃ infusion inhibits phosphate reabsorption in the proximal convoluted tubule by a mechanism not dependent on PTH or volume expansion, but apparently correlated with intraluminal CO₂.