The goal of this study was to evaluate the functional behavior of recruitable (mostly capillary) and nonrecruitable (mostly arterioles and venules) components of the intramyocardial microvasculature in the in situ heart of intact experimental animals. For this purpose fast X-ray computerized tomography (CT) scans were performed in a group of anesthetized pigs. Each scan was performed during an aortic root angiogram and was repeated after sequential injections of a suspension of nonradioactive microspheres into a selected coronary artery. Regional myocardial perfusion (F, ml.g-1.min-1) and intramyocardial blood volume (rho, ml/g) were estimated from the dynamic CT image sequences. For the intramyocardial microcirculation, rho = AF + B square root of F [where A = 0.016 min and B = 0.076 (ml.min/g)1/2] was shown to describe the rho-to-F relationship over the entire range of flows observed. With increasing embolization with 15-microns diameter microspheres, the coefficients A and B changed in a way consistent with A describing the transit time through the functionally recruitable component and B/ square root of F describing the transit time through the functionally nonrecruitable component of the microcirculation.