The role of three threonine residues in the M2 hydrophobic region of the GABAA receptor has been investigated by replacing these polar residues with alanine at the 6', 10' and 13' positions of M2 in the GABAA alpha 1, and beta 1 subunits and co-expressing the mutated subunits in the baculovirus Sf9 insect cell system. GABA did not elicit a current in cells expressing either the 6' or 13' threonine to the alanine mutants. The mutant subunits formed intact heteromeric GABAA receptors as judged by the binding of [3H] muscimol or the relative level of alpha 1 protein present in the plasma membrane. In contrast, a chloride current was generated by GABA in cells expressing the 10' mutant receptor. However, the current decayed more rapidly to baseline in the continued presence of GABA in the 10' mutant receptor than in the wild type receptor. The results are discussed in terms of the possible roles of the threonine residues in the ion conduction pathway.