The blood pressure lowering effect of urotensin I, a vasodilatory peptide obtained from urophyses of a bony fish species (Catostomus commersoni) was studied by tail plethysmography in conscious normotensive and in spontaneously hypertensive rats. Urotensin I (10 mU/100 g body weight i.v.) lowered the blood pressure, depending on the preinjection level. The lowering of pressure amounted to 12.4 +/- 1.28% of the preinjection level in normotensive rats, 17.9 +/- 1.46% in hypertensive females and to 28.9 +/- 1.06% in hypertensive males. The differences between the normotensive and each group of hypertensive animals were statistically significant (p less than 0.05 to p less 0.001). The blood pressure-lowering effect of urotensin I was less pronounced in female hypertensive rats than in hypertensive males of the same age. No significant difference was found in the preinjection blood pressure level between the group of younger (169-176 days) and older (247-249 days) male hypertensive rats. However, the blood pressure in the younger rats was significantly more affected by urotensin I than that in the older rats (p less than 0.05).