A highly specific monoclonal antibody directed against nitric oxide-tryptophan (NO-W) with good affinity (10-9 M) and specificity was developed. In the rat brain, using an indirect immunoperoxidase technique, cell bodies containing NO-W were exclusively found in the intermediate and dorsal parts of the lateral septal nucleus. No immunoreactive fibres were found in the rat brain. This work reports the first visualization and the morphological characteristics of cell bodies containing NO-W in the mammalian brain. The restricted distribution of NO-W in the rat brain suggests that this molecule could be involved in specific physiological mechanisms.