The present study was performed to evaluate the nephron site of inhibition by calcitonin of phosphate reabsorption in the thyroparathyroidectomized rat. Pharmacologic doses of salmon calcitonin markedly inhibited fluid and phosphate reabsorption by the superficial proximal tubule. However, continued phosphate reabsorption between the superficial late proximal and early distal tubule, as well as along the distal tubule, blunted the phosphaturic effect of calcitonin. We conclude that the phosphaturic effect of a pharmacologic dose of salmon calcitonin is primarily due to an inhibition of fluid and phosphate reabsorption by the proximal convoluted tubule.