The aging process as currently understood is intimately connected with formation of new collagen ( 1,2,3,4), and modification of existing elastic tissue ( 5,6,7,8,9). However, the studies have involved principally aorta and skin; little is known about above processes of other tissues. In previous work on connective tissue obtained by implantation of polyvinyl sponges (Ivalon) *, collagen content/g of sponge implanted, increased with age of tissue( l o ) . Collagen formed rapidly during first 2 or 3 weeks after implantation of sponge and then much more slowly. The present study was undertaken to determine in various tissues of rat, the relation of age and sex to content of soluble protein, soluble collagen, insoluble collagen and elastin. Such data would show whether tissues other than aorta and skin exhibit comparable changes with age.Methods. Male and female rats, Wistar strain, of 3-5 weeks, 8 months and 2 years of age were used. Samples (01.1-1 .O g) of tail tendon, aorta, skin, uterus, lung, muscle, heart, liver, kidney and spleen were homogenized in 10 ml saline in Virtis "23" homogenizer for 5 minutes. Each homogenate was centrifuged at 20,000 x g for % hr and supernatant (saline soluble protein) was decanted. The residue was suspended with occasional stirring, in 20 ml 0.1 N NaOH and remained at 2 5 " over night. The resulting mixture was centrifuged a t 30,0010 x g for % hr and the supernatant removed. The residue was extracted a second time with 101 ml 0.1 N XaOH for 2 hr a t 25" and centrifuged as above. The 2 extracts, which contained the alkali-soluble protein (containing soluble collagen) were then combined. The above saline soluble and alkali-soluble protein fractions were together designated "soluble protein'' (Fraction I).The residue was treated with 10 ml 0.1 N NaOH a t 100" for 15 min, the mixture centri-
