Effects of cetyltrimethylammonium and two other n-alkyltrimethylammoniums on the permeation of benzoic acid through excised guinea pig dorsal skin were examined. Cetyltrimethylammonium markedly increased both the flux and permeability coefficient in the concentration range of 0.5 and 5 mm. However, 50mM cetyltrimethylammonium decreased them. The presence of 2 mm cetyltrimethylammonium, which induced a maximal enhancement effect, also increased the fluxes of 4-methyl, 4-ethyl and 4-n-propyl substituents of benzoic acid, but the enhancement effects were less. Analysis of the free energy of transfer of the methylene group of the substituents from the aqueous phase to skin suggested that cetyltrimethylammonium made the skin relatively more hydrophilic. Electron spin resonance spectra analysis by a long-chain quaternary alkylammonium analog, 4-(N,N-dimethyl-N-pentadecyl)ammonium-2,2,6,6-tetramethylpiperidine-1-ox yl iodide (CAT-15), showed that the spin label was present in a nearly solid state in both excised skin and its stratum corneum. This finding suggested the high affinity of the long-chain alkylammoniums to proteins in the stratum corneum and the involvement of the interaction between the cationic surfactants and the proteins in their improvement of the hydrophilic property of the skin, as well as their marked enhancement effects on the skin permeation of relatively hydrophilic drugs.