The effect of a novel enzyme (PreR-Co) that activates renal prorenin was studied on rabbit aorta with and without endothelium. It was tested in: I) The basal tone of non stimulated or angiotensin II (Ang II)-sensitized rings or precontracted with norepinephrine (NE) or PGF₂ or high KCL concentration, II) In rings pretreated with enalaprilat or losartan or PD 123319 or L-NAME or HOE-140 or indomethacin or serine protease inhibitors (PMSF, aprotinin, SBTI), Kallilkrein and bradykinin were also tested in Ang II-sensitized rings. PreR-Co produced a vasorelaxant effect in the basal tone and in precontracted rabbit aorta. The effect was endothelium independent and potentiated by endothelium removal or NO synthase inhibition and abolished by boiling the enzyme. Besides, the effect improved when basal tone was increased in Ang II-sensitized aortic rings or in precontracted vessels. No activation of the Ang II, bradykinin, prostaglandin or NO pathway mediating PreR-Co response could be obtained suggesting a direct action of the enzyme. This action seems to be dependent of esterasic activity since serine protease inhibitors like PMSF and aprotinin were able to block the vasorelaxant effect of PreR-Co.