Strips of bovine mesenteric arteries, brought to sustained contraction by addition of 3.0 microM phenylephrine, relaxed during exposure to shortwave light. The action spectrum of the photorelaxation was characterized by most effectiveness of relaxation in the range of 360 to 400 nm; on the contrary, shortwave light of 280 to 300 nm increased the tension. The photo-induced relaxation was accompanied by an increase in the cGMP level, measured 30 sec after the onset of radiation, and the action spectrum of the increase in cGMP seemed to coincide fairly well with the action spectrum of relaxation. Both the increase in cGMP and relaxation in response to increasing light intensity at 400 nm fitted the rectangular hyperbolic equation. The K values (the intensity which gave half maximal response) for cGMP increase and relaxation, respectively, obtained by non-linear least square regression analysis, were found to assume very close values (1.3 mW/cm2 for cGMP increase and 1.5 mW/cm2 for relaxation). The action spectrum of the crude soluble guanylate cyclase (GC) activity displayed a peak around 400 nm. Our results suggest that there is a close association between photo-induced increase in cGMP and relaxation, probably as a result of interaction between shortwave light and the heme moiety of GC.