The diameter of subepicardial capillaries was measured in stop-motion photo-micrographs of normoxic rat hearts. Mean diameter over the whole cardiac cycle was 4.41 μm (0.09, SEM). Calculations indicate that mean diameter during systole is about 4 μm and during diastole is about 5 μm. The deformability of rat erythrocytes was evaluated by aspirating the cells into micropipets of various diameters. All cells traversed a 2.8-μm pipet at a mean ΔP of 0.17 mm Hg and a 2.5-μm pipet at a ΔP of 2.9 mm Hg. Below 2.5 μm, the pressure required to aspirate 100% of the cells increased linearly as the channel diameter decreased and reached 104 mm Hg at 1.9 μm. Comparison of deformability data with frequency distributions of coronary capillary diameter indicates that all cells traverse all capillaries during diastole and traverse most superficial capillaries during systole. In the subendocardium, however, systolic tissue pressure is very high relative to erythrocyte deformability. Consequently, perfused capillaries should be compressed to the minimum thickness of an erythrocyte (about 1.8 μm). Calculated pericapillary O 2 gradients demonstrate that such narrow capillaries cannot sustain aerobic metabolism throughout the tissue. This is particularly true since capillary compression impedes erythrocyte entry, and thereby increases functional intercapillary distance. We conclude that: (1) Compression and narrowing of capillaries during systole can account for the transmural gradient in tissue pO 2. (2) During diastole, capillary dimensions are perfectly matched to the dimensions and deformability of erythrocytes.