Extracts of the carp urophysis elicit a marked decrease in urine flow in the anaesthetized hydrated rat. Reproducible dose-dependent responses are obtained within the range of 2 to 16 μg of acetonedried carp urophysis per 100 gBW of the rat. The carp urophysial antidiuretic substance is peptidic, and is different from the neurohypophysial peptides. The bulk of antidiuretic activity is located in the electrondense granules in the carp urophysis. The antidiuretic substance, probably urotensin I, is found generally in teleost urophyses. The activity per mg of acetonedried urophysis is higher in freshwater teleost species than in seawater species.