The mode and mechanism of action of Sefsol-318, a medium chain glyceride and a potent percutaneous absorption enhancer, on the in vitro permeation of papaverine hydrochloride through hairless rat skin were investigated and compared with those of laurocapram (Azone). The total amount of the drug delivered through excised skin over 28 h from aqueous solutions of the drug in which 5% S-318 or Azone was suspended was about 820 or 420 times higher, respectively, than from the solution alone. Experiments using liposomes as models, indicated that both the enhancers markedly increased the fluidity of lipid membranes. Skin conductance measurements in hairless rats indicated that they both also increased in vivo skin moisturizing and water holding capacity. These results suggest that the mechanism of action of Sefsol-318 and Azone in enhancing skin permeation are similar. But following in vitro pretreatment of the excised skin with 5% Sefsol-318 and aqueous emulsion for 2 h, skin permeation of papaverine hydrochloride through the pretreated skin was much lower than through non-treated skin in the presence of Sefsol-318. In contrast, the enhancing effect of Azone on the pretreated skin was similar to that of Azone on the in vitro non-treated skin. We found that, unlike Azone, Sefsol-318 disappeared from skin completely one day after 24 h-in vivo pretreatment of skin with aqueous gels containing each agent. In agreement, drug permeation through skin excised one day after the in vivo pretreatment with Sefsol-318 was almost the same as in non-pretreated controls without Sefsol-318. This difference in the mode of action of Sefsol-318 and Azone may arise from the difference in the residence times of these enhancers in skin.