Micropuncture studies were performed in intact and thyroparathyroidectomized (TPTX) dogs to evaluate the relationship between the absolute level of serum ultrafilterable phosphate concentration (SUFP) and proximal bicarbonate reabsorption. In both groups of animals, a reverse linear relationship was noted between the serum phosphate level and the tubular fluid bicarbonate concentration (TFHCO3). For the intact dogs, TFHCO3=32.27 SUFP+/-2.17, r=0.715, p=0.02, and for the TPTX dogs, TFHCO3=35.68-2.39 SUFP+/-2.26; r=0.619, p less than 0.02. Furthermore, when late-proximal tubular values for absolute bicarbonate reabsorptive rate (THCO3)/100 nl single nephron filtrate were plotted against SUFP, a direct relationship was found for both groups; intact dogs: THCO3=205 SUFP-169+/-136, r=0.707, p less than 0.05; TPTX dogs: THCO3=224 SUFP-882+/-221, r=0.625, p less than 0.01. Variations in plasma bicarbonate concentration or PCO2 did not contribute to the relationship between SUFP and bicarbonate reabsorption. The data suggest that extracellular fluid phosphate concentration is an important modulator of proximal bicarbonate transport, independent of the action of parathyroid hormone.