The goal of our study was to determine a contribution of nNOS to the increase of brain NO synthase activity induced by chronic N-acetylcysteine (NAC) treatment. Young 4-week-old male Wistar Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) were subjected to treatment with NAC (1.5 g/kg/day) for 8 weeks. At the end of experiment total NOS activity was determined in the brainstem and cerebellum with and without specific nNOS inhibitor S-methyl-L-thiocitrulline (SMTC, 10(-6) mol/l) by measuring the formation of L-[(3)H] citrulline from L-[(3)H] arginine. Chronic NAC treatment had no effect on blood pressure (BP) of WKY, while it attenuated BP increase in young SHR. Total NOS activity was increased in the brainstem of SHR compared to WKY, but this strain difference was abolished by SMTC. Chronic NAC treatment of SHR increased total NOS activity by 32% in the brainstem and by 67% in the cerebellum. After the incubation of brainstem and cerebellum with SMTC there were no significant differences in NOS activity of NAC-treated rats compared to strain-matched controls. Taken together, nNOS seems to be responsible for the increase of total NOS activity in the brain of SHR. SMTC inhibited 86% and 70% of NAC-induced increase of total NOS activity in the brainstem and cerebellum, respectively. Thus, nNOS is responsible not only for strain differences but also for NAC-induced increase of total NOS activity in the brain.