We prepared a series of five esters of 6-dimethylaminohexanoic acid, and characterised the compounds by their NMR and IR spectra. Their ability to function as transdermal penetration enhancers was subsequently evaluated using excised human skin as a membrane, modified Franz diffusion cells, and theophylline as a model permeant. The penetration-enhancing efficiency of esters 1-5 was studied in the donor media of propylene glycol and isopropyl myristate, and expressed as the corresponding enhancement factors (EF). All the esters increased the penetration of theophylline through the skin. The enhancement factor for the most active substance, undecyl 6-dimethylaminohexanoate, was 118.5 (+/- 19) from propylene glycol.