The possibility of a plasma oxygen diffusion barrier implies a significant resistance to oxygen diffusion and the existence of capillary erythrocyte-associated transients of oxygen. This effect was analyzed by measuring intracapillary blood and tissue pO(2) in the hamster chamber window model using a noninvasive intravital microscopy palladium porphyrin phosphorescence decay technique for two set light excitations (high and low). Using high light excitation, intracapillary blood pO(2) was 13.7 +/- 6.1 mm Hg, and increased to 18.0 +/- 4.5 mm Hg for low light excitation. For high light excitation, intracapillary blood pO(2) peaks were in the range of 25-30 mm Hg, and the lowest values were in the range of 5-10 mm Hg. Reducing the excitation provided a more uniform pO(2) ranging 15-25 mm Hg. With temporal reduction in blood capillary pO(2), levels were correlated to the increase in phosphorescent amplitude that corresponded to plasma gaps. Tissue pO(2) measured at low light excitation in the proximity of capillaries was 23.1 +/- 1.8 mm Hg. In conclusion, low intracapillary blood pO(2) measurements at full hematocrit are an artifact, only observed when oxygen consumption by the measurement technique was excessive and/or absorption of the excitation light was increased by the absence of RBCs. These findings suggest that resistance to oxygen diffusion in plasma is a minor factor in tissue oxygenation by capillaries in the hamster model.