The role nitric oxide (NO) and oxygen radical species (ROS) in regulating capillary perfusion was studied in the hamster cheek pouch model during normoxia and after 20 min of exposure to 10% O₂ and 90% N₂. We measured PO₂ by using phosphorescence quenching microscopy and ROS production in the systemic blood. Identical experiments were performed after treatment with the NO synthase inhibitor L-NMMA, and after the reinfusion of a NO-donor (DETA-NO) following treatment with L-NMMA. Hypoxia caused a significant decrease in the systemic PO₂. During normoxia arteriolar intravascular PO₂ decreased progressively from 47.0 ± 3.5 in the larger arterioles to 28.0 ± 2.5 mm Hg in the terminal arterioles, conversely intravascular PO₂ was 7 - 14 mm Hg and approximately uniform in all arterioles. Tissue PO₂ was 85% of baseline. Hypoxia significantly dilated arterioles, reduced blood flow, increased capillary perfusion (15%) and ROS (72%) relative to baseline. Administration of L-NMMA during hypoxia further reduced capillary perfusion to 47% of baseline and increased ROS to 34% of baseline, both changes being significant. Tissue PO₂ was reduced by 33% vs. the hypoxic group. Administration of DETA-NO after L-NMMA caused vasodilation, normalized ROS, and increased capillary perfusion and tissue PO₂. These results indicate that during normoxia oxygen is supplied to the tissue mostly by the arterioles, while in hypoxia oxygen is supplied to tissue by capillaries by a NO concentration dependent mechanism that controls capillary perfusion and tissue PO₂ involving capillary endothelial cell responses to the decrease in lipid peroxide formation controlled by NO availability during low PO₂ conditions.