Morphological and contractile characteristics of individual myocytes isolated from rats of 2, 6-9, and 24-25 mo of age were measured. The average myocyte length measured under high power light microscopy in unattached cells increased from 133 microns at 2 mo to 146 microns at 6-9 mo to 162 microns at 24-25 mo of age. The average slack sarcomere length was 1.85, 1.83, and 1.82 microns at 2, 6-9, and 24-25 mo, respectively. The average cell volume measured via Coulter counter techniques approximately doubled between 2 and 24 mo. During the electrically stimulated twitch in individual unloaded myocytes contracting from slack length the absolute amplitude of cell shortening increased with age, but when expressed relative to cell length or as sarcomere shortening the age effect was obliterated. Neither the maximal velocity of myocyte shortening (cell length/s) nor the calculated maximal sarcomere shortening velocity varied with age. The time course of the twitch increased with aging, due largely to an increase in the time to peak shortening. Thus aging is associated with an increase in cell size due to the addition of sarcomeres. Except for a prolonged time course, the twitch contraction characteristics normalized for cell length and sarcomere number in unloaded ventricular myocytes contracting over a range of sarcomere lengths below the slack length do not change appreciably with age.