Peak force and membrane potential were recorded from papillary muscles and trabeculae excised from the ventricles of adult rat hearts. Experiments were performed at 2.5 mM Ca2+ and 26 degrees C. In thick preparations (diameter 0.2-1.2 mm) an increase of stimulation frequency caused a reduction of peak force and action potential duration as has been found in many studies previously. In thin preparations (diameter less than 0.2 mm) both peak force and action potential duration were almost independent of stimulation frequency. When the flow of Tyrode solution through the muscle bath was reduced an increase of stimulation frequency caused a reduction of peak force and action potential duration in thin preparations. We conclude that the reduced peak force and action potential duration in papillary muscles at high stimulation frequencies is due to insufficient exchange of metabolites and oxygen between the medium and the core of the muscle. The results indicate that the critical diameter for the preparations is about 0.2 mm.