We tested the relaxant effect of increased protamine sulfate (PS) amounts (10, 20, 50, 100 and 150 μg/ml) on the isolated mesenteric arteries of normotensive and spontaneously hypertensive (SH) rats, with or without endothelium. PS caused concentration-dependent relaxation of isolated mesenteric arteries in both types of rats. The relaxation effect of PS was lower in SH rats than in normotensive ones. Our results indicate that the vascular smooth muscles play a significant role in PS-mediated relaxation.