Gillichthys urotensin II (GUII; 1.5-150 nmol kg-1) reduced blood pressure in anesthetized rats, diastolic pressure being reduced to a greater extent than systolic. The effect was slow in onset (peak at 4-5 min) and longlasting (up to 60 min), and was accompanied by a reflex tachycardia. The hypotension was not blocked by propranolol or by mepyramine. In pithed rats, GUII (150 nmol kg-1) reduced the pressor responses to sympathetic nerve stimulation, noradrenaline, and Arg-vasopressin. Somatostatin (150 nmol kg-1) had no effect on the sympathetic pressor response but reduced heart rate. It is concluded that GUII is a vasodilator in the rat, an action not shared with somatostatin.