More than 110 derivatives of alkoxycinnamic acids were synthesized and their hypolipidemic activities were evaluated in a screening system with rats. Cinnamic acids, alpha-methylcinnamic acids, and their various esters with a higher p-alkoxy substituent were found to possess hypolipidemic activities higher than or comparable to that of clofibrate. The proper length (C12--C16) and the para position of the alkoxy substituent seem to be essential for activity. Chloroethyl and methacryloxyethyl esters and monoglycerides of some of the active p-alkoxycinnamic acids were more active than the corresponding free acids.