Release of endothelin-1 (ET-1) from the mesenteric arteries of spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats were measured by radioimmunoassay after purification using immunoaffinity column. The identity of ET-1 in perfusate was established using reversed-phase high-performance liquid chromatography. The mesenteric arteries from 5- to 6- and 9- to 10-week-old SHR released a significantly higher level (mean +/- SEM) of ET-1 (32.8 +/- 2.8 and 47.5 +/- 4.1 pg/h, respectively) than WKY age-matched rats (23.4 +/- 2.1 and 34.8 +/- 3.7, respectively). There was an age-related increase in ET-1 release in both groups of rats. Though the SHR studied were in the hypertensive stage, the present results suggest that locally produced ET-1 may contribute to the development of hypertension independent of the plasma levels.