The effects of six monovalent anions (chloride, bromide, iodide, nitrate, perchlorate, and thiocyanate) on [3H]strychnine binding to glycine-gated chloride channels were examined. These anions have previously been shown to permeate glycine-gated chloride channels and stimulate [3H]strychnine binding. Whereas low concentrations (10-200 mM) of all these anions enhanced [3H]strychnine binding, higher concentrations (0.2-3 M) of thiocyanate, perchlorate, and iodide produced a robust inhibition of radioligand binding, and a more modest inhibition was observed with the same concentrations of nitrate and bromide. The presence of one binding site for anions at glycine-gated chloride channels can account for either the activation or the inhibition phase, but not both. However, these biphasic effects can be explained by the presence of two binding sites for anions at these channels. Two models with two anion binding sites were considered, the first assuming both allosteric activation and inhibition of the binding of the ligand, and the other explained by allosteric activation combined with competitive inhibition. Mathematical expressions for both models were formulated, and the equations obtained yielded satisfactory fitting to the results obtained with all anions tested in both concentration-response and saturation experiments. These equations also permitted the calculation of several parameters describing the interaction of the anions with these channels. The main difference in the behavior of these anions relates to the extent to which they produce activation of [3H]strychnine binding and to their cooperative interaction at the two putative anion binding sites. Thus, a strong negative cooperativity was observed for the simultaneous binding of two molecules of chloride, bromide, or nitrate, but not for the simultaneous binding of thiocyanate, perchlorate, or iodide. This latter property may be related to the conductance of these anions through glycine-gated chloride channels.