Using immunohistochemistry and in situ hybridization the distribution of nitric oxide synthase (NOS) was investigated in the rat brain during pre- and postnatal development At E15 weak NOS-like immunoreactivity (NOS-LI) could be seen in the differentiation held of the anterior hypothalamus. At E17 strong NOS-LI was observed in the developing neurons of the hypothalamic paraventricular nucleus, supraoptic nucleus, anterodorsal nucleus and lateral hypothalamic areas. In the thalamic paratenial nucleus a strong NOS-LI was observed in these neurons at E17, E18 and P1 with a weaker intensity at P3, P7, P9 and P15, whereas at P30 and in adult rats no NOS-positive neurons could be detected. NOS expression at E17 and P3 was verified by in situ hybridization. These results suggest that NO may have a developmental role at least in one of the regions studied, the thalamic paratenial nucleus.