SUMMARYThe length and width of rat and ferret ventricular myocytes have been measured using a linear photodiode array; the volume of the myocytes has been calculated based on the assumption that the cells were elliptical cylinders. During a twitch contraction, there was a decrease in cell length, but no significant change in the calculated cell volume, because the cells increased in width. Inotropic interventions not only resulted in a greater shortening of the cell during each contraction, but also a greater increase in cell width. Changes in cell length, width and volume on changing the osmotic strength of the bathing solution have also been investigated. The increase in volume in hypotonic solution, and the decrease in hypertonic solution, were the result of changes in the cell width; there were no significant changes in the cell length. It is concluded from the latter experiment that the lateral compliance of a cell is greater than its longitudinal compliance, and, therefore, during a twitch contraction, when the cell shortens, the displacement of the cell contents from the two ends of the cell and the expansion of the cell laterally will not act as a large force to oppose shortening.