1. The effect of alinidine on transmural flow redistribution during hypoperfusion was studied in the isolated rat heart. 2. Low flow perfusion of paced (5 Hz) rat Langendorff preparations with Tyrode's solution for 60 min resulted in loss of left ventricular pressure amplitude and contracture. 3. In the presence of 45.6 mumol/L alinidine in the hypoperfusion medium contractility was maintained and contracture was missing. 4. Myocardial sphere densities were assessed by infusion of fluorescent microspheres after 60 min of hypoperfusion. Histomorphometric evaluation of the left ventricular wall indicated marked redistribution of myocardial flow to the epicardium. Endo/epicardial quotients of sphere densities were significantly greater in alinidine-treated hearts. 5. Perfusion of hearts with 45.6 mumol/L alinidine for 30 min at a pressure of 80 cmH2O decreased coronary flow and contractility, but did not change the transmural quotient of sphere densities. 6. These results show that in isolated rat hearts with a ventricular wall of only 3-4 mm thickness severe hypoperfusion induces a shift in transmural flow distribution which is prevented by alinidine.