A number of factors which may complicate studies dealing with the calcium metabolism of teleosts are pointed out. Thus, low environmental levels of calcium markedly affect both gill RNA and sodium metabolism in dilute sea water, and the absence of monovalent cations affects calcium metabolism in dilute environments. Hypophysectomy is not always followed by the disappearance of cortisol from the plasma, and prolactin—which is required for the survival of either hypophysectomized or intact animals in a divalent ion-free environment—stimulates cortisol production. It is also pointed out that Fundulus kansae can be maintained in divalent ion-free environments for long periods, and that calcium is mobilized from internal reservoirs such as acellular bone and scales. Failure to survive in such environments is due to the regulatory effect of environmental calcium on sodium metabolism rather than on calcium metabolism per se. Finally, it is pointed out that while one parameter of calcium metabolism—such as plasma calcium levels in male fish—may remain constant throughtout the year, other parameters such as calcium uptake may vary considerably. Seasonal differencees in responses to mammalian calcitonin have also been noted.