Recent studies on urotensins I and II and experiments on urophysial function are described. Chemical and pharmacological studies showed that: (i) The isoelectric points of urotensins I and II are pI 5.5 ± 0.05 and 5.9 ± 0.05, respectively; (ii) two active peaks of urotensin I are obtained by gel chromatography of HCl-extracted urophyses on Bio-Gel P6, one corresponding to a substance of mol wt 4000, the other, mol wt 2000; (iii) the rat-hypotensive activity of urotensin I affects peripheral blood vessels at an undetermined receptor site(s) and is not mediated by cholinergic, adrenergic, or histaminergic receptors or the corresponding transmitter substances; and (iv) urotensins I and II increase blood pressure and urine flow in the American eel (Anguilla rostrata). Urophysial content of urotensin II in the white sucker (Catostomus commersoni) was high during about 3 months preceding spawning and was considerably lower during the remainder of the year. Pituitary content of isotocin is significantly higher in male suckers after spawning as compared with the content before spawning. The reverse was found on urophysial content of urotensin II in the male fish.