SummaryAdenosine, adenosine triphosphate, inosine triphosphate and uridine triphosphate were administered iv in 100 mg doses to intact and adrenalectomized dogs to test the ability of these compounds to increase the plasma volume. Only adenosine triphosphate was found to be effective. It readily induced marked elevation (av 20.3% rise) of the plasma volume of dogs possessing adrenal glands but had no effect on the plasma volume of the dog lacking adrenal cortices unless injected simultaneously with small (25 mg) but ineffective doses per se of the glucocorticoid prednisolone. The combined injectate promptly raised the plasma volume an average of 17.1% above control values based upon experiments on 14 adrenalectomized dogs.