The effects of aging on mechanical performance of isolated canine right ventricular trabeculae were studied in two age groups. The first group was comprised of nine dogs, about 9 months of age. The second group was composed of seven dogs over 8 years of age. Aging had no significant effect on developed force. Extent of shortening tended to decrease. There was a significant decrease in both the rate of rise of tension and the velocity of shortening (20 per cent). This reduction was primarily due to an increased duration of contraction. Twitch duration increased by as much as 40 per cent during aging but most of this prolongation was due to a profound slowing of relaxation. Aging caused a significant increase in passive stiffness since equivalent changes in muscle length brought about twice as much increase in resting tension in the aged muscle as in the young muscle. On the other hand, aging caused a significant shift of Lo to the right. Taken collectively, these results indicate that aging is associated with increased passive stiffness and decreased speed of contraction without changes in strength. © 1978.