The renal effects of neurohypophysial hormones in fishes and amphibians are discussed. Injections of arginine vasotocin (AVT) elicit diuresis in fishes, but antidiuresis in amphibians. However, the physiological significance of these hormonal responses remains to be demonstrated. Studies with bioassays and radio-immunoassays on circulating levels of AVT indicate that hypovolemia may be a very potent stimulus for the release of the hormone. Hyperosmotic stimuli may not be as important. In anurans, mesotocin has a glomerular diuretic effect. This neurohypophysial hormone appears to dilate, while AVT constricts, the afferent glomerular circulation in bullfrogs, Rana catesbeiana.