Changes in coronary perfusion pressure lead to alterations in intracoronary myocardial volume that may be associated with regionally altered microvascular morphology. Transmural variations in coronary capillary geometry were quantified as a function of coronary perfusion pressure in glutaraldehyde-fixed canine hearts. Capillary volume fractions, diameter, numerical density, anisotropy, and sarcomere length were measured using computer analysis of light microscopic images of sections taken transverse or longitudinal to the muscle fiber axis. Capillary volume was 4-6% of myocardial wall volume and exhibited a significant transmural gradient, increasing from epicardium to endocardium. Vessels 4 to 5 microns in diameter with a density of 2900 mm-2 appear to increase in diameter and alter their cross-sectional shape with increasing pressure, rather than increasing in number, suggesting an effective distensibility of approximately 0.007 mm Hg-1. Quantification of vessel anisotropy was directly related to cross-sectional shape and demonstrated that the capillaries are highly oriented. These findings indicate that intramyocardial capacitance is at least in part associated with nonhomogeneous changes in coronary capillary morphology with altered perfusion pressure.