The effect of solar irradiation on ex vivo dermatomed hairless rat skin samples maintained in culture on flow-through diffusion cells for at least 24 h was evaluated by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide assay and by histological observations. Transepidermal water loss (TEWL) measurements and kinetic analysis of the permeation of both tritiated water and ¹⁴C caffeine through the skin were performed after full-spectrum solar exposure involving the use of a xenon arc solar simulator. After a UV exposure of less than 420 mJ/cm², skin integrity and permeation of both water and caffeine did not change significantly. In contrast, after a 420 mJ/cm² UV exposure, the epidermis appeared more contracted, associated with an increase of 55% of TEWL and 220% of the skin permeation of tritiated water after 6 h. The data suggested a dramatic alteration of the skin barrier integrity. Moreover, the flux of ¹⁴C caffeine increased rapidly by 338% of the absorption of water 12 h after irradiation. These results reveal the presence of a threshold UV exposure that would not modify skin penetration.