An analogue of colchicine, beta-lumicolchicine, does not bind tubulin or disrupt microtubules. However, this compound is not pharmacologically completely inactive. beta-Lumicolchicine was found to competitively inhibit [3H]flunitrazepam binding and to enhance muscimol-stimulated 36Cl- uptake in mouse cerebral cortical microsacs. It also markedly potentiated GABA responses in Xenopus oocytes expressing human alpha 1 beta 2 gamma 2S, but not alpha 1 beta 2, GABAA receptor subunits; this potentiation was reversed by the benzodiazepine receptor antagonist flumazenil. These results strongly suggest a direct effect of beta-lumicolchicine on the GABAA receptor/chloride channel complex and caution that it possesses pharmacological effects, despite its inability to disrupt microtubules. Furthermore, beta-lumicolchicine is structurally unrelated to benzodiazepines or quinolines and may provide a novel approach to the synthesis of ligands for this receptor.