The possible coexistence of the two non-adrenergic, non-cholinergic (NANC) inhibitory neurotransmitters, adenosine 5'-triphosphate and nitric oxide in the myenteric plexus was investigated using whole-mount preparations of rat ileum, proximal colon and anococcygeus muscle. The presence of adenosine 5'-triphosphate in neurones was examined using the quinacrine fluorescence technique. After localizing and taking photographs of quinacrine-fluorescent neurones and nerve fibres, the same tissues were then fixed and processed for NADPH-diaphorase activity, a marker for nitric oxide-containing neurones. We have demonstrated for the first time that almost all quinacrine-fluorescent myenteric neurones in the proximal colon are also NADPH-diaphorase reactive, while only a subpopulation of quinacrine-fluorescent neurones in ileum and anococcygeus muscle were also NADPH-diaphorase reactive.