We studied the effect of acute oral phosphate loading on parathyroid hormone (PTH) secretion and action. Eighteen adults were studied before and after ingestion of 1 g elemental phosphorus as neutral sodium-potassium phosphate in grape juice or in water and 1.7 g NaCl in juice was used as a control. Five subjects were studied after ingesting 1.5 g P in juice. Blood was drawn every 15 min from -45 to +300 min for measurement of serum ionized calcium (Ca++), P, total Ca, creatinine, and immunoreactive PTH (iPTH); urine was collected hourly. Serum P declined after NaCl ingestion but rose significantly after ingestion of 1 g P in juice or in water. There were no significant changes in serum Ca++, iPTH, or urinary cyclic AMP excretion. Acute ingestion of 1-1.5 g P, which exceeds the P content of a normal meal, does not decrease serum Ca++ or increase iPTH secretion or renal action in normal adults.