SUMMARY.— The incorporation of 35SO4 into the acid mucopolysaccharides of skin in vitro has been used to study the metabolism of these substances in skin showing gross actinic elastosis, in exposed skin with less severe actinic changes, and in non-exposed skin. Whereas no alteration in the pattern of incorporation was demonstrated in the epidermis, elastotic dermis showed an increased incorporation of 35SO4, especially into the chondroitin-6-sulphate peak, as compared to the non-exposed dermis. The possible biological implications of these results are discussed, namely, (a) that treatment with weak alkali produces a greater yield of sulphated acid mucopolysaccharides from elastotic dermis than from normal dermis, or (b) that these substances are depolymerized in elastotic dermis with alteration in their elution positions on ion-exchange resin chromatography, or (c) that a real increase in turnover of chondroitin-6-sulphate exists when elastin is being laid down.