Measurement of local tissue PO2 using recessed microcathodes has again been criticized. Therefore, we reexamined electrode performance. Sharply beveled electrodes (3 micron external diameter) were fabricated with several tip recess lengths (4-10 micron), and some recesses were filled with hydrated polymer. In vitro, 2-mm agar (3%) sheets were equilibrated with solution of known PO2 (continuously flowing). Electrode currents at 100-micron intervals through the agar and of convected superfusion solution were compared. At the longest recess lumen length-to-diameter ratio of 10, minimum response midagar (1 mm) averaged 98%. Performance improved with the use of recess polymer and increased recess length. For in vivo studies, microcathodes (ratio approximately 10) were fluid calibrated, and PO2 was measured at 10-20 micron through canine femoral artery walls. PO2 distribution fit a model for radial diffusion with medial O2 consumption. After local cyanide application to the femoral wall, PO2 fit a model for radial diffusion without tissue O2 consumption. Carefully designed microcathodes and experiments measure accurate tissue PO2.