The efficacy of the methyl esters of medium chain n-alkyl fatty acids as penetration enhancers was evaluated in vitro using various animal and human skins with minoxidil as the test drug. Both methyl nonanoate and methyl caprate at a 10% concentration were found to be effective penetration enhancers for a 2% solution of minoxidil in alcohol USP. The percent of the applied radioactive dose of minoxidil penetrated after 17 h was 5-8 times greater for methyl non-anoate and methyl caprate enhanced solutions than for a 2% solution of minoxidil in alcohol USP alone or with the addition of 10% Azone, dimethylsulfoxide (DMSO) or N,N-diethyl-m-toluamide (DEET). The penetration enhancing activity of methyl caprate was effective for human, mouse, and hamster skins. Methyl caprate also enhanced the penetration of vitamin D3, erythromycin, triamcinolone acetonide, testosterone, and hydrocortisone.