We investigated whether nitric oxide (NO) may serve a role in bladder function by immunohistochemical analysis of the distribution of intrinsic NADPH-diaphorase and functional study of isometric tension recordings via a photo-induced adequate nitric oxide (PIANO) generating system using rat bladder. Results suggest that a small number of NADPH-diaphorase-positive perikarya are present within the bladder wall and within adjacent small ganglia. Furthermore, NADPH-diaphorase-positive nerve fibers were observed in the adventitial and muscular layers, subjacent to the urothelium and perivascular fibers. Rat bladder strips precontracted with 3$mu$M carbachol were reversibly relaxed upon NO generation by UV irradiation. PIANO-mediated relaxation was sensitive to oxygen free radicals. In addition, tissue cGMP levels were increased by the PIANO generating system and elevated cGMP levels were decreased by pretreatment of guanylate cyclase inhibitor, methylene blue. These results indicate that NO may serve a role in modulating bladder tone in the rat.