Both the methyl ester of β-carboline-3-carboxylic acid and the 6, 7-dimethoxy-4-ethyl derivative of this compound are potent convulsants in rodents, while the ethyl ester of β-carboline-3-carboxylic acid does not cause convulsions, even when administered at very high doses. The rate of degradation of these compounds by rat plasma (in vitro) parallels their potencies as convulsants. In contrast, 3-carboethoxy-β-carboline was found to potently elicit tonic and clonic convulsions in the squirrel monkey (Saimiri sciureus). Furthermore, the rate of degradation of 3-carboethoxy-β-carboline in monkey plasma (in vitro) is negligible compared with rats. No significant differences were observed in either the potency or efficacy of GABA to inhibit [3H] β-carboethoxy-β-carboline binding in rat and monkey brain. These data strongly suggest that pharmacokinetic, as well as pharmacodynamic, factors may determine the pharmacologic profile of these β-carboline-3-carboxylic acid esters.