A technique to measure small vessel pressures and diameters in the right ventricle of a beating rabbit heart was evaluated. The method allowed free motion of the heart while performing these measurements. This was accomplished with a strobed light source synchronized with the heart. Thus, the observer "sees" the vessels only at the same point in the cardiac cycle. Slowly advancing the strobe pulse allowed visualization of an entire cardiac cycle. The pressure measurements were performed using an electromechanical micromanipulator which was programmed to synchronize the motion of a micropipette to the motion of a small vessel. For comparison, another technique was also utilized which fixed a portion of the right ventricle and allowed motion to be centered around this fixed point. Pressures in veins using the free motion and fixed techniques were compared and found to be statistically different (5.8 or minus 0.7 compared to 13.5 plus or minus 4.7 mmHg, P is less than .01) with the free motion technique recording the lower values. The phasic relationship between venous diameter variation and left ventricular pressure also was determined. This relationship was variable between venous networks but quite consistent within a network. The relationship between vessel pressure and vessel diameter revealed significant declines in pressure in relatively large vessels (less than 140 micrometers). This free motion technique can be utilized to provide information concerning not only the normal physiology of the coronary circulation, in terms of pressure distributions and the effects of extravascular pressure, but also the changes in vascular pressure which occur in ischemic myocardium and other tissues with inherent motion.