Cerebral autoregulation maintains cerebral blood flow (CBF) despite marked changes in mean arterial pressure (MAP). Sodium nitroprusside (SNP) reduces blood pressure by vasodilatation but is reported to lower CBF, probably by a reduction in its perfusion pressure. We evaluated the influence of SNP on CBF and aimed for a 20% and then 40% reduction in MAP, while keeping MAP ≥ 50 mmHg, to challenge cerebral autoregulation. In 19 healthy men (age 24 ± 4 years; mean ± SD) duplex ultrasound determined right internal carotid (ICA) and vertebral artery (VA) blood flow. The SNP reduced MAP (from 83 ± 8 to 69 ± 8 and 58 ± 4 mmHg; both P < 0.0001), total peripheral resistance, and arterial CO₂ tension (P _aCO²; 41 ± 3 vs. 39 ± 3 and 37 ± 4 mmHg; both P < 0.01). Yet ICA flow increased with the moderate reduction in MAP but returned to the baseline value with the large reduction in MAP (336 ± 66 vs. 365 ± 69; P = 0.013 and 349 ± 82 ml min^-1; n.s.), while VA flow (114 ± 34 vs. 112 ± 38 and 110 ± 42 ml min^-1; both n.s.) and CBF ((ICA + VA flow) × 2; 899 ± 135 vs. 962 ± 127 and 918 ± 197 ml min^-1; both n.s.) were maintained with increased cerebrovascular conductance. In conclusion, CBF is maintained during SNP-induced reduction in MAP despite reduced P aC O2 and the results indicate that SNP dilates cerebral vessels and increases ICA flow. Journal compilation