At any location in a respiring tissue, partial pressure of oxygen (PO2) is influenced by the local oxygen consumption rate. Consumption rates in vascular tumor tissues have previously been estimated for macroscopic regions. Using oxygen electrodes, we measured PO2 profiles across microregions (87 microns to 286 microns wide) of tumors (R3230AC mammary adenocarcinoma) in a rat dorsal skin flap preparation and mapped adjacent microvessels. By comparing measured PO2 values with theoretical simulations, we deduced local consumption rates. Results for six profiles ranged from 0.83 to 2.22 cm3 O2/100 g/min. The mean (+/- SD) was 1.52 +/- 0.51 cm3 O2/100 g/min. This technique permits investigation of variations in consumption at a microregional level.