The objective of the present study was to investigate the expression of neuronal nitric oxide synthase (nNOS) mRNA in stress-related areas after restraint. Male Wistar rats (n=4-6/group) submitted to 2 h of restraint during one (acute) or seven (chronic) days were sacrificed 24 h after the last restraint period. In situ hybridisation was performed with oligonucleotide probes radiolabeled with (35)S. Acute restraint induced a significant increase in nNOS mRNA in the paraventricular hypothalamic nucleus (PVN), medial parvocellular part, dorsolateral periaqueductal grey (DLPAG) and medial amygdaloid nucleus, but not in the hippocampal formation. This effect persisted after chronic restraint in the PVN and DLPAG. These results suggest that restraint stress induces changes in gene expression of nNOS in areas related to stress reactions.