Regulatory control of the hypothalamic–pituitary–adrenocortical axis (HPA) originates principally from the paraventricular nucleus (PVN). PVN contains a substantial population of nitric oxide synthesizing (NOS) neurons. In the present study, the effect of immobilization stress over these neurons was investigated by means of the NADPH–diaphorase histochemical technique. A considerable increase in the number of NADPH-diaphorase reactive neurons was observed following acute immobilization of rats. Our data showed that NO activity in rat’s PVN was significantly affected by acute immobilization stress. This suggests an important role of this part of the brain and NO-ergic system in anti-stressor system which main role is to process strategies for coping with different types of stress and to restore the disrupted homeostasis.