SummaryThe ability of topically applied retinoic acid to improve photoaged skin has stimulated research interest into its mechanism of action. Currently available assay systems are eitherin‐vitroor mouse models, neither of which are truly representative of thein‐vivosituation in man. Another drawback is that skin biopsies available from studies using retinoic acid to treat photoageing are of insufficient size to accomplish biochemical and molecular assays. In order to address these problems, a 4‐dayin‐vivoretinoid assay has been developed which serves as a good predictive model for the chronic effects of retinoic acid on skin and helps to characterize the mechanism of action of retino