Intravital microscopy of the superfused cremaster muscle was used to measure the density, diameter, length, hematocrit, red cell velocity, and red cell flux in capillaries of the pentobarbital-anesthetized hamster. Oxygen microelectrodes were used to measure oxygen tension (Po2) at a position 75-100 micrometers deep in the muscle between the venous ends of capillaries and, very importantly, at the superfusate-muscle interface. These parameters were measured in resting and contracting muscles and under three values of superfusate Po2: low (8mm Hg), medium (40 mm Hg), and high (75 mm Hg). These data were complete enough to be useful input parameters in a recently developed mathematical model of oxygen transport in exposed tissue (A. S. Popel, 1981, Math. Biosci. 55, 231-246). The model indicated that with high superfusate Po2, oxygen was supplied to the resting muscle almost exclusively from the superfusate because of the vasoconstriction and reduced blood flow. Oxygen consumption of the resting muscle was estimated to be 0.4 ml O2/100 ml tissue X min, assuming muscle oxygen consumption was uniform and independent of Po2 above 1 mm Hg. The estimated rise in oxygen consumption with exercise was four to eight times resting muscle values, which agrees with previously published data. Also, the model predicted an inlet capillary Po2 of 27 mm Hg with a low superfusate Po2, which is consistent with the few available direct measurements. The model emphasized that with measurement of the Po2 at the superfusate-tissue interface, the complex O2 transport effects of the superfusate can be accurately characterized. Measurement of this and other parameters of the model leads to a potentially useful prediction of the Po2 distribution within tissues under a variety of conditions.