A new category of signalling molecules – transmitter gases – has appeared. Nitric oxide (NO) is generated by nitric oxide synthase (NOS) and diffuses as a short-lived transcellular messenger through the plasma membrane. NADPH-diaphorase (NADPH-d) is a marker enzyme for NO-producing neurons. In this study the pattern of NADPH-d stained neurons in Diphyllobothrium dendriticum is described and compared to the pattern of aminergic and peptidergic neuronal elements and to that of the musculature stained with TRITC-labelled phalloidin. NADPH-d staining was observed in neurons in the bilobed brain and along the 2 main nerve cords and in nerve fibres close to the body musculature and the musculature of the reproductive ducts, the walls of the testicular follicles and in sensory endings in the tegument. The NADPH-d staining reaction and the 5-HT or FMRFamide immunoreactivities occur in separate sets of neurons.