1. The influence of perfusion pressure on flow distribution in the left ventricular wall was investigated with a newly developed microsphere method in Langendorff preparations of the rat heart. 2. Microspheres with a diameter of 10 microns stained with the fluorescent dye yellow-green were infused into the perfusion medium. The hearts were then relaxed with a calcium-chelator, fixed with formaldehyde and cut transversally with a freezing microtome. Photographs were taken of the 52 microns slices with a fluorescence photomacroscope. The ventricular wall was subdivided into four zones, evaluated planimetrically and the beads were counted for calculation of microsphere density. 3. Perfusion of groups of hearts, paced at 5 Hz, with Tyrode's solution at pressures of 30, 50, 80 or 120 cmH2O for 30 min revealed an increase of left ventricular pressure amplitude, coronary flow, myocardial oxygen consumption and flow redistribution towards the endocardium of the left ventricular wall with increasing perfusion pressure. 4. The quotient of the sphere densities in the inner endocardium over that in the outer epicardium, however, was already maximal at a pressure of 80 cmH2O and amounted to 1.59 +/- 0.15. This quotient was only 0.52 +/- 0.11 at a perfusion pressure of 30 cmH2O, thus indicating massive change in flow distribution upon hypoperfusion. 5. These results indicate that transmural left ventricular flow redistribution in response to hypoperfusion in the isolated rat heart is similar to that in hearts of much larger species.