Several esters of beta-carboline-3-carboxylic acid were synthesized and tested in respect to their affinity for the benzodiazepine receptor in bovine cortex membranes. Out of these derivatives, the methyl, ethyl, and n-propyl ester were clearly the most potent, while the n-butyl, benzyl, and 3-pyridylmethyl ester were considerably less active. Moreover, several beta-carboline-3-carboxylates with ethanol derivatives as ester alcohol components were all less active than the ethyl or n-propyl ester themselves. It is concluded that the affinity of beta-carboline-3-carboxylates to the benzodiazepine receptor is profoundly dependent on molecular size, as well as hydrophobic and electronic parameters of the ester alcohol component.