Five choline esters, lauroylcholine, myristoylcholine, palmitoylcholine, stearoylcholine and oleoylcholine, were evaluated as skin penetration enhancers by testing their effects on the penetration of six drugs, acyclovir, 17 beta-estradiol, hydrocortisone, nitroglycerin, all-trans-retinoic acid and trifluorothymidine, across hairless mouse skin in vitro and comparing the results to those obtained with oleic acid. The results show that the transdermal delivery of the drugs tested from propylene glycol vehicle systems, can be significantly increased by adding small amounts of choline esters and/or oleic acid to the vehicle. Lauroylcholine was a better enhancer than oleic acid for the transdermal delivery of 17 beta-estradiol and, in mixtures, lauroylcholine and oleic acid acted as synergists giving larger enhancement of the transdermal delivery of nitroglycerin and acyclovir than when used separately.