It has been suggested that pericardial fluid functions as a lubricant rather than a means of transmitting pericardial pressure from one region of the heart to another. Since the functional behavior of pericardial fluid depends on fluid thickness, we measured pericardial volume and fluid distribution. In seven animals, we found that the normal canine pericardium contains 0.25 +/- 0.15 ml of pericardial fluid per kg of body weight, resulting in an average pericardial fluid thickness of only 0.34 +/- 0.27 mm. We next determined the pericardial fluid distribution in eight anesthetized mongrel dogs (17-29 kg). Color video images were recorded, while green dye (0.1 ml) was injected into the pericardial space overlying the ventricular apex to allow visualization of the pericardial fluid distribution. Within 26 +/- 17 s (range 15-53 s), dye reached the base of the heart. After 15 min of equilibration, the dye distribution appeared very nonuniform with dye accumulation over the interventricular and atrioventricular grooves. Little or no dye was present over the right and left ventricular free walls. We conclude that pericardial fluid thickness over the interventricular and atrio-ventricular grooves is sufficient to allow fluid motion in these regions. In contrast, pericardial fluid thickness overlying the ventricular free walls is very thin. Thus, in these regions the pericardial fluid functions primarily as a lubricant; and regional variations in pericardial pressure may occur.