Histochemical and ultrastructural studies have already demonstrated that chronic exposure to UV radiation induces profound alterations in all structural elements of the skin and that topical all-trans retinoic acid (tRA) can substantially correct much of the tissue damage. However, previous biochemical studies on dermal components of the extracellular matrix have led to contradictory results, particularly with regard to the effect of chronic UV exposure. The aim of our study was to investigate changes in collagen content and other dermal modifications induced by tRA in irradiated and non-irradiated hairless mouse skin. Hairless mice were exposed to increasing doses of UVB for 10 weeks (the cumulative total dose was 4.6 J cm-2). After the UV irradiation period the animals were treated with 0.05% tRA or with ethanol-polyethylene glycol vehicle alone three times a week for up to 10 weeks. Non-irradiated animals underwent the same treatments. The main clinical and histological changes induced by UVB exposure were erythema, wrinkling, keratosis and epidermal thickening. Following UVB exposure, tRA treatment did not improve the clinical aspect but increased the width of the dermal repair zone. Fibronectin, laminin and type I and VI collagens were detected by indirect immunofluorescence techniques in this zone. Type I and III collagens were quantitated in skin fragments after cyanogen bromide digestion and polyacrylamide gel electrophoresis. Under our experimental conditions, UVB irradiation alone induced neither changes in total collagen nor in type I and III collagen levels. tRA treatment of irradiated skin significantly increased both type I and III collagen levels by factors of 1.33 and 1.88 respectively. The ratio of type III to types I + III increased significantly. Topical tRA also increased collagen type levels in non-irradiated hairless mouse skin. Type I collagen increased proportionally to type III. This study leads to the conclusion that topical tRA exerts direct or indirect effects on collagen metabolism in irradiated as well as non-irradiated hairless mouse skin.