GABAA receptor function was studied in outside-out patches from guinea pig hippocampal neurons using a drug application system with an exchange time of under 1.5 ms. Application of GABA to these patches induced a Cl- conductance that desensitized with prolonged exposure. Increasing GABA concentrations induced larger conductance increases that were associated with more complex patterns of desensitization. Smaller GABA responses desensitized with monophasic kinetics, whereas large responses displayed bi- and triphasic kinetics. Desensitization of the response to 1 mM GABA was triphasic in about 70% of the patches (tau = 15.4, 207, and 1370 ms) and biphasic in about 30% of the patches (tau = 44 and 725 ms). All phases of desensitization reversed at the Cl- equilibrium potential. Over the concentration range from 3 microM to 3 mM, both the rate and the extent of desensitization increased; however, complete desensitization was rarely observed. The increase in desensitization rate was due to an increase in the relative contribution of the faster phases with increasing GABA. The time constants of the three phases were independent of concentration. The different phases are not mediated by separate receptor populations, because double pulse experiments demonstrated interconversion among the fastest phase and the two slower phases. We demonstrate the plausibility of a model in which multiphasic desensitization is a consequence of the faster association rate at higher GABA concentrations.