Phosphate excretion in the laying hen (Gallus domesticus).

1. The excretion of phosphate during the calcifying and non-calcifying phases of egg formation was studied.2. The pattern of phosphate excretion was observed during increasing body loads of phosphate.3. A tubular mechanism for phosphate excretion is proposed in which tubular phosphate secretion occurs proximal to, or coincidental with reabsorption.4. Parathyroid extract increased the rate of phosphate excretion and appeared to inhibit both the reabsorptive and secretory mechanisms.5. The pattern of excretion of phosphate loads by hens in the calcifying phase was closely similar to that of birds in the non-calcifying phase when treated with parathyroid extract.6. The relationship between shell calcification and the parathyroids is discussed.