1) Radiation of UV light did not affect both arterial rings in resting tension.

2) Both arterial preparations contracted with various vasoconstrictors (KCI,NE,PE,BayK 8644 and (+S202 etc.) were relaxed by UV light radiation in a radiation time-dependent fashion.

3) The magnitudes of the relaxation were not significantly different in both the rings with or without intact endothelium.

4) MB and LY markedldy reduced the UV light-induced relaxation in both the rings.

5) PP significantly attenuated the UV light-induced relaxation of rabbit thoracic aorta, but did not affect that of porcine coronary artery.

6) The UV Light-induced relaxation of porcine coronary artery was significantly attenuated by DT or NF, while that of rabbit thoracic aorta was not.

7) (+)S202 significantly potentiated the UV light-induced relaxation of porcine coronary artery contracted with KCI or Hist.

Above results suggest that the UV light-induced relaxation of vascular smooth muscles is independent on the endothelium, and the relaxation results from primarily activation of guanylyl cyclase and is in part related to adenylyl cyclase and calcium metabolism. In adddition, a dihydropyridine calcium agonist, (+)S202, may sensitize vascular smooth muscle to the relaxing effect of UV light through some unknown mechanism.