Application of water and glycerin is known to influence skin mechanics. The kinetics of these processes are of great interest. A study was performed to show the immediate changes in skin-mechanics. A Dermaflex machine (R) was used to study 23 healthy volunteers. Water or glycerin was applied to the flexorside of the forearm, and readings were made after 3, 6, 9, 12 and 15 min. Regional untreated skin served as baseline. In agreement with earlier studies both substances influenced hysteresis. Water caused a significant increase in hysteresis after 12 and 15 min of hydration (P<0.01). Glycerin caused significantly increased hysteresis after 3 min (P<0.05) and the effect continued to the end of the observation period. No significant differences were seen in the distensibility. The onset of action is rapid for both substances, and the effects are therefore supposed to take place in the outermost layers of epidermis. The effect of glycerin on the hysteresis is more rapid in onset than that of water. Comparing the permeability coefficients, the effect on the mechanical properties of the skin does not appear to be determined by the permeability coefficients as water has a higher permeability coefficient but induces smaller changes than glycerin. Water alone does not appear to be the optimal plasticiser of human skin and other substances soluble in both water and lipids may have an even greater influence on skin mechanics in vivo.