Experiments were performed in male Sprague-Dawley rats (150-200 g). A silver clip (0.2 mm internal diameter) was placed on the left renal artery of rats. After operation the rats were divided into 4 groups sham group, 2K1C (two-kidney one clip) group, 2K1C+Arg (2K1C and L-Arg 150 mg/kg x d(-1) by drinking) group, and 2K1C+NAME (2K1C and L-NAME 10 mg/kg x d(-1) i.p.) group. The animals were studied at two time points (4 and 7 weeks after operation) corresponding to phases I and II of Goldblatt hypertension. The animals were deeply anesthetized with pentobarbital and perfused by the cardiac route with saline (100 ml) and freshly prepared 4% paraformaldehyde in phosphate buffer (300 ml). The caudal medulla was removed, then placed in 25% sucrose in PB for 12 h in a 4 degrees C fridge. The 40 microm coronal slices of brainstems were cut with cryostat, collected in PBS for nNOS study by immunohistochemistry. The results showed that (1) only a few neuronal nitric oxide synthase (nNOS) positive neurones were found in caudal medulla, including the depressor area of the ventral surface of medulla oblongata (VSMd) and the caudal pressor area (CPA) of the sham operated animals. The number of nNOS positive neurons in caudal medulla was significantly increased in 2K1C Goldblatt hypertension rats at 4 and 7 weeks. (2) The number of nNOS positive neurons in VSMd and CPA were 2K1C+Arg > 2K1C >2K1C +NAME > sham. (3) L-Arg enhanced the expression of nNOS whereas L-NAME inhibited the expression of nNOS in caudal medulla. (4) The character of nNOS expression was similar in Goldblatt hypertension rats at 4 weeks with that of the rats at 7 weeks.