The vasopressor responses of three natural endothelins (ET-1, ET-2, ET-3) and their precursors, pro-ETs, were studied in a pithed rat preparation, which allows the profile and the potency of vasoconstrictor agents to be determined in the absence of central control of the cardiovascular system. ET-1 was found to be 4- and 8-fold more potent in raising blood pressure than ET-2 and ET-3, respectively. The immediate precursors of these isopeptides, h-pro-ET-1 (human), p-pro-ET-1 (porcine), pro-ET-2 and pro-ET-3, produced significantly smaller pressor responses than their respective ETs, when measured either as peaks or as areas under the time-effect curve. Hence, the bioavailability of h-pro-ET-1, p-pro-ET-1 and pro-ET-2, assessed on the basis of these two parameters, was approximately 50% of that of their corresponding ET, whereas the bioavailability of pro-ET-3 was only 25% that of ET-3. Phosphoramidon inhibits metallopeptidases, the enzymes that convert pro-ETs to ETs. The approximate i.v. doses of phosphoramidon reducing by 50% the pressor effects of the pro-ETs were 2.5, 0.625, less than 2.5 (this dose produced 75% inhibition) and 5 mg/kg i.v. for h-pro-ET-1, p-pro-ET-1, pro-ET-2 and pro-ET-3, respectively. In conclusion, these results indicate that the rat may have more than one pro-ET converting enzyme, each specific for one of the natural pro-ETs studied, although the alternative explanation, that there is a single enzyme with different affinities for these pro-ETs, cannot be entirely dismissed.