In this work we have examined the appearance and distribution of nitric oxide synthase (NOS), with histochemical, immunohistochemical and biochemical methods, during development of the sea bass (Dicentrarchus labrax) gut. The data showed that both the calcium-calmodulin dependent neuronal isoform (nNOS) and calcium-independent inducible isoform (iNOS) are present in the larval gut of sea bass. The nNOS-immunoreactivity was present in the epithelial cells and enteric nerve cells of gut both in the 8-day-old specimens and in the 24-day-old-larvae. In the adult nNOS-immunoreactivity disappeared from epithelial cells, remaining in the wall intramural neurons and fibers. The iNOS-immunoreactivity was present in the epithelial cells of 24-day-old-larvae and was not detectable in the adult gut. Western blot analysis and determination of NOS activity also demonstrated the presence of the two NOS isoforms, nNOS and iNOS, in the gut of 24-day-old specimens. The presumably different roles played by the two isoforms of enzyme are discussed. The presence of nNOS isoform in the gut enteric neurons of the same larval stages of D. labrax in which we previously demonstrated the presence of substance P and Vasoactive Intestinal Polypeptide (VIP), may suggest that all these three components of the motility control system are already present in the larval phase. Nitric oxide (NO) may be also involved in the early immune response. The present results on the occurrence of iNOS isoform in epithelial gut cells of the same regions in which the gut-associated lymphoid tissue (GALT) will differentiate, may suggest for NO a role in early defence mechanisms, before the establishment of immune responses in GALT. Finally, the developmental and regional differences in nNOS and iNOS expression also suggest a regulatory role in development and differentiation of the sea bass gut.