Background: Autocrine and paracrine factors produced within the kidney regulate the cortical and medullary blood perfusion (CBP and MBP, respectively), which include endothelins, prostaglandins, reactive oxygen species, and nitric oxide (NO). This study investigated the role of NO in regulating the CBP and MBP of a kidney in both stroke-prone spontaneously hypertensive rats (SHRSP) and Wistar rats. Materials and Methods: Groups (n = 10 for each) of male SHRSP and Wistar rats (250-300 g) were prepared for experiments. Two laser Doppler microprobes were inserted, 1.5 and 4.0 mm, into their kidneys to measure CBP and MBP before and after the intrarenal infusion of L-NAME, the NO synthase inhibitor at dose of 10 μg/kg/min. At the end of the experiments, the animals were killed with an anesthetic overdose. Data ± standard error of the mean were subjected to Student's t-test and significance taken at P < 0.05. Results: Interstitial infusion of L-NAME into the corticomedullary border (CMB) causes significant reduction in MBP in both SHRSP and Wistar rats by 18% ±5% and 12% ±4%, respectively. The magnitude of reduction is closely similar in both strains. Acute infusion of L-NAME into CMB has no effect on CBP but increases the blood pressure (BP) in both strains equally (P < 0.05). Conclusion: These results suggest that NO plays an important role in regulating the tone of medullary blood vessels in both hypertensive and normotensive states with similar extent. L-NAME can easily spread into the systemic circulation, which is evidenced by the increase of BP. © 2018 Journal of Natural Science, Biology and Medicine | Published by Wolters Kluwer - Medknow.