M icropuncture studies were performed in the dog to examine the relationship between sodium and phosphate transport in the proximal tubule. In hydropenic, thyroparathyroidectomized animals, administration of parathyroid extract, saline, or acetazolamide resulted in a fall in proximal tubule fluid-to-plasma (TF/P) inulin ratio as well as a rise in tubule fluid-to-plasma ultrafilterable (TF/UF) phosphate ratio. A correlation was found between the changes in fractional reabsorption of sodium and phosphate but the phosphate changes were generally greater than those of sodium. Also, a high distal phosphate delivery in the face of low fractional excretion of phosphate in the urine in thyroparathyroidectomized dogs suggests significant phosphate reabsorption in the distal nephron. On the other hand, calcium chloride infusion to saline-loaded, normal dogs to suppress endogenous parathyroid hormone reduced proximal TF/UF phosphate without change in TF/P inulin, while both parameters remained unchanged in saline-loaded, thyroparathyroidectomized dogs after calcium infusion. An increase in proximal TF/UF phosphate associated with unchanged TF/P inulin was also demonstrated by administration of highly purified parathyroid hormone to saline-loaded, thyroparathyroidectomized dogs. It was concluded that although proximal tubule phosphate transport is generally closely related to that of sodium, the two can dissociate under certain experimental conditions, especially under the influence of parathyroid hormone. These observations also indicate that the effect of parathyroid hormone on proximal tubule phosphate transport is not solely dependent upon its effect on sodium transport.