The effect of lanthanum ion (La3+) on gamma-aminobutyric acid (GABA)-mediated Cl- currents was examined in the alpha 1 beta 2 or alpha 1 beta 2 gamma 2 subtype of GABAA receptors expressed in a human kidney cell line (A293), using a whole-cell configuration of patch-clamp techniques. La3+ dose-dependently stimulated the Cl- currents in the alpha 1 beta 2 gamma 2 subtype with an EC50 of 21.3 +/- 3.5 microM with a maximal potentiation of 240 +/- 16% as normalized to the GABA response at 5 microM. In the alpha 1 beta 2 subtype, however, the ion marginally potentiated GABA response, a maximal stimulation being less than 70% with an EC50 for La3+ near 200 microM. The stimulation of GABA response by La3+ in the alpha 1 beta 2 gamma 2 subtype was due to a decrease in the half maximal concentration for GABA and was more pronounced at the negative membrane potentials. This selectivity of La3+ toward the subtypes of GABAA receptors contrasts to that of Zn2+ which inhibits the currents in the alpha 1 beta 2, but not in the alpha 1 beta 2 gamma 2 subtype (Neuron, 5: (1990) 781-788). It appears that these polyvalent cations are useful in understanding the molecular basis for the functional diversity and in characterizing the molecular organization of native GABAA receptors.