Background: Insulin mediates endothelium-dependent vasodilation in peripheral tissues such as skeletal muscle and liver primarily via nitric oxide synthase (NOS), an effect reported to be greater in the skeletal muscle of women compared to men. Whether cerebral blood flow (CBF) increases in response to insulin or differs between the sexes remains unknown. We hypothesized that: a) insulin would increase CBF; b) CBF response to insulin would be NOS-mediated; and c) CBF response to insulin would be greater in women via greater NOS-mediated vasodilation. Methods: Healthy normal weight young women (n=6; BMI 22±1 kg/m2; age 23±2 yrs) and men (n=6; BMI 22±2 kg/m2; age 22±2 yrs) underwent two magnetic resonance imaging (MRI) study visits with randomized intravenous infusion of saline or the NOS inhibitor L-NMMA (loading: 5 mg/kg/min; maintenance 1 mg/kg/min). Pseudo-continuous arterial spin labeling (ASL) quantified global and regional cerebral perfusion. Scans occurred at baseline and at 60 minutes during an oral glucose tolerance test (OGTT; insulin-glucose challenge). Significance was determined using a mixed three-way ANOVA (Time x Condition x Sex) and was set at p<0.05. Results: Results are mean±SD. OGTT increased glucose and insulin (both p<0.05; no interaction). Mean arterial pressure and end tidal CO2 were similar at baseline and did not change during OGTT (both p>0.05; no interaction). Women tended to have higher heart rate (HR; 62±6 vs. 58±7 BPM; sex p=0.05), and HR increased with OGTT (both sexes Δ5±2 BPM; p<0.05). Women displayed greater global perfusion compared to men (women 43±5 men 35±4 mL/100g/min; sex p<0.05) and a similar pattern was observed regionally in the frontal, temporal, parietal, and occipital lobes (sex p<0.05). L-NMMA decreased global perfusion (saline 40±7 vs. L-NMMA 38±7 mL/100g/min; condition p<0.05) and perfusion to the frontal and parietal lobes (condition p<0.05). Despite the main effect of L-NMMA, during OGTT, global and temporal lobe perfusion decreased with saline but increased with L-NMMA (interaction; both p<0.05). Conclusions: Women exhibited greater global and regional perfusion and NOS inhibition reduced global perfusion similarly between the sexes. Surprisingly, some regions increased perfusion with NOS inhibition during OGTT. Therefore, insulin-glucose surge interacts with NOS in a regionally-specific manner to alter cerebral circulation, but this effect appears similar between sexes.