Oxygen-derived free radicals have been implicated in many important functions in the biological system. Electrical field stimulation (EFS) causes arterial relaxation in animal models. We found that EFS applied to neither muscle nor nerve but to Krebs solution caused a relaxation of rat aorta that had been contracted with phenylephrine. In the present study, therefore, we investigated the characteristics of this EIRF (electrolysis-induced relaxing factor) using rat isolated aorta. Results indicated that EIRF acts irrespective of the presence of endothelium. EIRF shows positive Griess reaction and is diffusible and quite stable. EIRF-induced relaxation was stronger on PE-contracted aorta than on KCl-contracted one, and inhibited by the pretreatment with methylene blue. Zaprinast, a cGMP-specific phosphodiesterase inhibitor, potentiated the EIRF-induced relaxation. NG-nitro-L-arginine, NO synthase inhibitor, did not inhibit the EIRF-induced relaxation. Deferroxamine, but not ascorbic acid, DMSO potentiated the EIRF-induced relaxation. These results indicate that electrolysis of Krebs solution produces a factor that relaxes vascular smooth muscle via cGMP-mediated mechanism.