A histochemical study of the distribution of NADPH diaphorase activity in the hypothalamus of normal rats was carried out. Our study demonstrates the presence of NADPH-diaphorase activity in the circularis and anterior and posterior fornicals nuclei for the first time. Additionally, we confirm the presence of NADPH-diaphorase-stained neurons in the paraventricular (both magno- and parvicellular neurons) and supraoptic nuclei, as well as a population of isolated positive neurons (not included in any hypothalamic nuclei) located among the different nuclei. Because NADPH diaphorase has recently been shown to be a nitric oxide synthase, our study reveals a wide presence of this enzymatic activity in the hypothalamus of the rat.