Experiments designed to study the degeneration of DNA in the retinas of aging rabbits are continuing on schedule; they are entering the fourth year of a study that is proposed to last for seven years, i.e., the median life-span of the rabbit. These studies are complemented by others in which the optic tissues of rabbits were subjected to 60 Co gamma-irradiation when the rabbits were six weeks old, and in this case the fate of the irradiated retinal DNA is being followed throughout the subsequent life-spans of the animals. The latter studies in their turn are further complemented by experiments utilizing heavy-ion irradiation that are funded by NASA. In parallel with the retinal studies, we are also engaged in direct, and longitudinal, investigations on the in vitro lifespans of skin fibroblasts from animals at ages throughout the median life-span; and, whenever possible, the same rabbits are used for both the skin and CNS studies. The efficacies of the DNA rapair mechanisms in the skin fibroblasts will be studied as a function of both "in vitro" and "in situ" age. Similar studies on retinal DNA in situ have already been completed.