The relation existing in vivo between the reservoir function of the stratum corneum following a 30-min application time and the total percutaneous absorption of molecules applied for a longer time was studied on hairless rats. One thousand nanomoles of 4 radiolabeled molecules (theophylline, nicotinic acid, acetyl salicylic acid, benzoic acid) were applied on 1 cm2 of back skin for 0.5, 2, 4, and 6 h. The total recovery in excreta and the whole animal body were measured 4 days later in order to determine the level of absorption. A direct proportionality (r = 0.99) between the total percutaneous absorption of these compounds and the duration of their application was observed. This suggests that a constant flux of penetration does exist in vivo. The reservoir function of the stratum corneum was measured by successive stripping of the treated area. Following a 30-min application, a strict correlation (r = 0.99) between the total amounts of molecules penetrated in 4 days and the amounts found in the reservoir of the horny layer was demonstrated. All together these findings show that the simple knowledge of the reservoir effect of the stratum corneum for a molecule applied for 30 min allows the predictive assessment of its total penetration resulting from various times of application.