Alveolar wall from the lung of aging humans shows a progressive decrease in maximal extensibility, which should follow an increase in resting tissue length rather than a reduction in maximal length. An increase in resting tissue length is compatible with the change in lung volumes and reduction in elastic recoil that occurs with time. A model of the lung was used to compare the effects of a change in resting tissue length in diminishing elastic recoil with that of a reduction in the volume density of the elastic elements (emphysema). Such differentiation is important in selecting an animal that may model the aging or emphysematous lung. In the rat, rabbit, and horse, alveolar walls show no decrease in maximal extensibility with age. In the male monkey (M. nemestrina and M. mulatta) between birth and 2.4 years there is a decrease in maximal extensibility that lacks significance for the limited age span examined. On the other hand, the energy loss in length-tension cycling (hysteresis) of alveolar wall increases in aging humans, diminishes in rats and rabbits, and shows little change in horses and monkeys. The breaking force of alveolar wall increases with age in rats and rabbits but does not change significantly in the other species. Of these species, the monkey promises a better model of the age-related changes in maximal extensibility of alveolar wall. A measure of maximal extensibility can distinguish the effects of dilatation of air spaces from those of destruction of alveolar wall in causing loss of lung elastic recoil.