The transition metal Zn2+ is differentially distributed in the central nervous system, where it is proposed to be a neuromodulator. One of the documented effects of Zn2+ is the antagonism of gamma-aminobutyric acid (GABA)-mediated synaptic inhibition. This antagonism is presumed to result from a direct interaction of Zn2+ with the GABA receptor/ionophore complex, although the characteristics of Zn2+ sensitivity are dependent on the particular GABA subunit combination. In this study, we examined the effects of Zn2+ on homomeric rho 1 GABA-activated channels expressed in Xenopus oocytes. Zn2+ was found to be a mixed antagonist of these recombinant rho 1 GABA receptors. The antagonism was predominantly competitive at low Zn2+ concentrations (< or = 100 microM), whereas at high Zn2+ concentrations (> 100 microM) a noncompetitive antagonism was apparent. Evidence is presented showing that the antagonism was not due to an interaction of GABA and Zn2+ in solution but, rather, resulted from interactions of these two ligands with the GABA-activated channel. A mechanism is proposed for Zn(2+)-mediated antagonism in which GABA and Zn2+ bind to distinct sites on the GABA complex. The apparent mixed antagonism may arise from different Ki values for the binding of Zn2+ to non-agonist-bound or agonist-bound receptors. However, two distinct Zn2+ binding sites, one competitive and one noncompetitive, could also give rise to the dual antagonism.