Neuronal nitric oxide synthase was located in various organs of the goldfish by NADPH-diaphorase histochemistry and immunohistochemistry. Positive cells were detected throughout the digestive tract. A particularly dense plexus of nitric-oxide-synthase-containing fibers was present at the opening of the pneumatic duct into the esophagus and at the intestinal sphincter separating the esophagus and the intestinal bulb. The nitroxergic innervation was mainly confined to the muscularis. The muscular layer of the swim bladder and of the pneumatic duct was densely equipped with stained neurons and fibers. In the heart, the majority of small neurons located at the sinu-atrial junction was found to be positive for nitric oxide synthase. The muscularis of the urinary duct was supplied by fibers originating from many intramural ganglia harboring intensely stained neurons. These results suggest that nitric oxide represents a widespread transmitter in the peripheral nervous system of teleost species.