An important feature of ligand-gated ion channels is their exquisite ability to discriminate between ions. Still, little is known about the mechanisms underlying, or structural determinates of, this ability. We examined the structural elements underlying the ionic selectivity of rho1 GABA receptors expressed in Xenopus oocytes and human embryonic kidney cells using site-directed mutagenesis and two-electrode voltage-clamp or patch-clamp techniques. The wild-type GABA receptor was chloride selective, with a small but significant permeability to potassium (PNa+ : PK+ : PCl- = 0 : 0.03 :1). Mutation of an alanine to glutamate at position 291 (thought to be located at the intracellular end of the second transmembrane domain), formed a channel that exhibited little discrimination among ions (0.70:0.87:1), while deletion of a neighbouring proline (290) was chloride selective, but had elevated cation permeabilities compared to the wild-type channel (0.12 : 0.14 : 1). Together, the two mutations (DeltaP290/A291E) caused a reversal of selectivity (2.72 : 3.59 : 1). We also examined the effects of neutralizing and reversing the charge of the adjacent, and highly conserved, arginine. Mutation of the neighbouring arginine to glutamate (R292E) increased the cation permeability similar to the DeltaP290/A291E double mutant (2.4 : 3.0 : 1), whereas neutral mutations at this position (R292M or R292C) retained chloride selectivity (0 : 0.11 : 1.0 and 0 : 0.14 : 1.0, respectively). Our experiments suggest that the effective charge near the presumed intracellular mouth of the pore is critical for ionic selectivity.