The effects of all-trans retinoic acid(t-RA)on photodamaged and normal non-irradiated skin were examined in hairless mice (Skh:HR-1). After being exposed to increasing doses of UVB for 10 weeks (total dose = 1.4 J/cm²), the animals were then treated with 0.1% t-RA in ethanol (50 μl, five times per week) for another 10 weeks. Several animals (the follow-up group) were further observed after the termination of the t-RA treatment to investigate if the t-RA effect was reversible. Wrinkle effacement induced by t-RA was compared with three other parameters: a) de novo collagen synthesis, b) width of the dermal repair zone, and c) epidermal thickening. Interestingly, t-RA did not stimulate collagen synthesis in animals not exposed to UVB. In the irradiated animals, the time course of wrinkle reduction correlated with the stimulation of collagen synthesis. After a synchronous initial lag phase of 4-6 weeks, the wrinkling decreased from the maximum grade of 4 to a mean grade of 1.3, whereas collagen synthesis was enhanced to 245% of the control at week 10 of t-RA treatment. In contrast, a similar lag phase was not observed for either the appearance of the dermal repair zone or epidermal thickening. In the follow-up group, upon termination of t-RA treatment, collagen synthesis returned to the control level. Wrinkle effacement and thickening of the dermal repair zone, however, did not regress, suggesting the anti-photoaging effect of t-RA was not reversible over this time frame. The correlation between the length of the lag phases for collagen synthesis and wrinkle reduction points to the possibility that collagen plays an important role in tRA-induced wrinkle effacement. Both parameters are thus important endpoints for investigating the mechanism of RA-induced repair of photodamaged skin.