The effect of ultraviolet (UV) light on taenia caeci smooth muscle of guinea-pig was studied. This stimulus induced a decrease in membrane conductance, hyperpolarization of the smooth muscle cells, a decrease in spike activity and a diminished level of cGMP and enhancement of lipid peroxidation. The hyperpolarization evoked by UV light was attenuated in the presence of sodium nitrite, reaching a maximum at 0.5 mM sodium nitrite. The level of cGMP was enhanced in the presence of nitrite and lipid peroxidation was not changed. Under these conditions lipid peroxidation was not changed during UV irradiation either. The UV-induced hyperpolarization was abolished in the absence of extracellular calcium, inhibited in the presence of 8-Br-cGMP and enhanced by an increase in the extracellular calcium concentration. These results are consistent with the supposition that UV-A irradiation blocks calcium influx via a cGMP-dependent mechanism.