SUMMARY
The amounts of ionized, complexed and protein-bound calcium and total inorganic phosphate were measured in the plasma of the eel. Groups comprised freshwater, seawater and distilled water eels; treatment with corticotrophin (ACTH), cortisol, aldosterone or oestrogen; adrenalectomized and hypophysectomized eels and those with the corpuscles of Stannius removed. In the intact animal, the amounts of ultrafiltrable and ionic calcium and inorganic phosphate in the plasma were very constant despite wide variation in the electrolyte composition of the external medium. In the freshwater eel, 1 week after removal of the corpuscles of Stannius, there was a rise in the level of ultrafiltrable calcium and a decline in that of inorganic phosphate. At 3 weeks after operation, although total calcium concentration remained elevated, ultrafiltrable and ionic calcium concentrations returned to control values. Adrenalectomy and hypophysectomy of the freshwater eel resulted in a decline in total calcium concentration with change in the levels of ultrafiltrable calcium or inorganic phosphate. In the seawater eel, adrenalectomy or removal of the corpuscles of Stannius resulted in an increase in total and ultrafiltrable calcium concentrations. Injection of ACTH, cortisol or aldosterone did not affect calcium concentrations but gave a slight decline in inorganic phosphate levels. Injection of oestrogen increased total calcium concentration without affecting ultrafiltrable or ionic calcium concentrations. The relationship of the amounts of ultrafiltrable calcium and inorganic phosphate to the solubility product [ultrafiltrable calcium] × [phosphate] is discussed and attention drawn to the importance of protein-binding in calcium homeostasis.