Endogenous and sun-induced aging of the skin cause distinct morphological alterations. In this study, we have analysed the ratio of collagen III to collagen III plus I in extracts of sun-exposed (face) and sun-protected (abdomen) aged skin, as well as in collagens synthesized by fibroblasts during in vitro culture derived from actinically damaged and sun-protected skin of other subjects (face, medial aspect of the upper arm vs. abdomen, lateral aspect of the forearm). Furthermore, the amount and extent of post-translational modifications of newly synthesized collagens were determined. Chronic sun exposure of the skin does not have an impact on the quantity of collagenous proteins newly synthesized in cell culture. The proportion of collagen III in pepsin extracts of sun-damaged skin is increased relative to sun-protected skin. However, fibroblasts derived from sun-exposed skin synthesize a lower proportion of collagen III than cells from sun-protected skin. The hydroxylation of lysyl residues in newly synthesized alpha 2(I) and alpha 1(III) collagen chains is reduced by UV irradiation, whereas hydroxylation of lysyl residues in alpha 1(I) chains and of prolyl residues in alpha 1(I), alpha 2(I) and alpha 1(III) chains is unaffected by UV irradiation. These data provide circumstantial evidence to indicate that collagen synthesis is influenced independently by endogenous and sun-induced aging.