We are developing a method to measure myocardial intravascular and extracellular volumes using digital subtraction angiography (DSA). The advantages over traditional methods are temporal resolution and repeatability. We first validated the method using phantoms placed in the field for DSA, along with a calibration wedge containing a solution of 1 part contrast agent (iohexol) to 6 parts water. Known amounts of this solution were added to the phantom. The volume of solution in the phantom was calculated using DSA by comparing the change in gray level in the phantom to the changes in gray level along the length of the calibration wedge after correction for scatter and veiling glare. DSA calculated volume agreed well with the known amount of contrast for volumes ranging from 5 to 40%. We have measured intravascular and extracellular volumes in isolated, perfused dog interventricular septa using different contrast agents, one of which remains intravascular and one which leaks into the interstitial space. We found values of intravascular and extracellular volumes of approximately 12 and 33 ml/100g, respectively, which agree well with published values. Being able to measure these volumes offers the opportunity to address important issues in biomechanics such as the effects of intravascular and interstitial volumes on the stress-strain properties of cardiac tissue.