Early biochemical studies of ultraviolet (UV) irradiated human skin reported a loss of insoluble collagen with a concomitant increase in the soluble fraction. Recent work has described an early increase in type III collagen during chronic irradiation of hairless mice as determined by cyanogen bromide digests of whole skin. In order to understand the correlation of these events and those seen with histochemistry, in the present study we irradiated hairless mice for up to 24 weeks with approximately 4 minimal erythema doses (MEDs) of UVB thrice weekly with Westinghouse FS-40 bulbs. Skin samples were taken at 4-week intervals from irradiated and age-matched control mice. Collagen was isolated from other skin proteins by acid extraction, pepsin digestion, and salt precipitation. Estimates of types I and III collagen were made by interrupted polyacrylamide gel electrophoresis and densitometric scanning. Compared with unirradiated controls, there was a small increase in the ratio of type III to total collagen after 8 weeks of UV. There were no significant increases at later time points until after 24 weeks of radiation. Total collagen in normal mouse skin, determined by hydroxyproline content, remained constant over the 24 weeks, while UV radiation produced significant increases at 4, 8, 12, and 16 weeks, returning to control levels at week 20. There was no change in the degree of hydroxylation at any time point in either group. Thus, chronic UV exposure resulted in increased collagen synthesis until late in the course of irradiation. Because there is a lack of consistent change in the ratio of type III to total collagen, the early increases in collagen content may represent both types I and III, synthesized in relatively unchanging proportions.