The source of phosphate (P) for the hyperphosphaturic effect of PTH is uncertain. Male CD rats (210 g) were subjected to bilateral ureteral cannulation and thyroparathyroidectomy. The next day parathyroid hormone (PTH), 25 U/rat or 100 U/rat, was given SC to half (vehicle to others). Plasma and urine samples were collected hourly for 4 hr and analyzed for Ca and inorganic P. Extracellular fluid (ECF) P was derived from plasma P data assuming ECF to be 20% of body weight. Passociated with the Ca released from bone by the calcium mobilizing effect of PTH was calculated by Assuming a 3:2 mole ratio of increased ECF Ca: released P. For both doses of PTH the ECF phosphate loss alone and the sum of the ECF phosphate loss and Ca-associated P each accounted for less than half the observed increase in urinary P. Therefore, the major source of P for the hyperphosphaturic effect of PTH had not been identified. Thyroparathyroidectomized rats were treated with 50 U PTH (or vehicle). Four hr later tissues were collected under ether anesthesia and were analyzed for inorganic P and perchloric acid soluble P. In the PTH-treated rats, there was a significant decrease in inorganic P in red blood cells, rectus abdominus muscle, and liver, but there were no significant changes in the inorganic P of spleen, kidney cortex, or small intestine. Changes in acid soluble P reflected only altered inorganic P content. Themagnitude of the response in muscle suggests thatmost of the extra P present in the urine after PTH treatment came from the soft tissues. (Endocrinology94: 1331, 1974)