Single myocytes were isolated from mouse, rat, guinea-pig, rabbit, sheep and human ventricle, and the amplitude and velocity of their unloaded contraction measured using a video camera and a system for measuring their length. Measurements were done at 32 degrees C in solutions containing either low calcium (1-1.3mM) or a concentration of calcium (usually 8mM) that produced a maximum amplitude of contraction. Unlike the cells of the majority of species, human cells could be successfully isolated only by using protease in addition to collagenase. To determine the effect of protease, guinea-pig cells were prepared with or without this enzyme. There was no significant difference in the characteristics of the cells prepared by either method. There was little species variation in the length of the cells or the length of resting sarcomeres. However, there were considerable species differences in the velocity of shortening and of relaxation. Myocytes from larger animals tended to contract and relax more slowly than those from smaller animals, although there was little systematic variation in the absolute amplitude of contraction. There was a significant correlation between the normalised contraction velocity of single cells and the average heart rate reported for that species. The trend for decreased relaxation velocity with lower heart rate did not reach statistical significance. Right ventricular cells from rat heart contracted more rapidly than those from the left ventricle, a finding similar to that reported for intact muscle strips. We conclude that species and left/right differences observed in the contraction velocity of whole muscle can be detected at the level of the single myocyte.