1. Regional distinctions in GABA type A (GABAA) miniature IPSC responses are thought to be determined by postsynaptic receptor composition. The kinetics of receptor activation and deactivation were studied using rapid exchange (100 micros) of GABA at excised patches containing recombinant (alpha1beta1gamma2 or alpha2beta1gamma2) and native (cortical) GABAA receptors. 2. Receptors activated by brief (< 1 ms) pulses of GABA demonstrated a characteristic current response, hereby referred to as the 'receptor system response'. System response properties included agonist concentration-dependent peak amplitudes and concentration-independent maximal rates of activation and deactivation. Receptor subtypes were characterized functionally and phenotyped using the system response characteristics. 3. System responses obtained for alpha1beta1gamma2 receptors exhibited a single phenotype while alpha2beta1gamma2 receptors exhibited either a predominant slow deactivation (type I) or a relatively infrequent faster (type II) phenotype. Receptor system responses of alpha2beta1gamma2 receptors reached peak currents twice as fast as those of alpha1beta1gamma2 receptors (0.5 versus 1.0 ms) but decayed 2 or 6 times more slowly (taulong of approximately 190 and 62 ms for type I and II alpha2beta1gamma2, and approximately 34 ms for alpha1beta1gamma2 receptors). 4. Receptor system responses from cultured fetal mouse cortical neurons could be statistically separated and classified into five major types with little intragroup variability, primarily based on variations in the current deactivation phases. 5. Receptors subjected to pharmacological modulation exhibited alterations in system response properties consistent with known mechanisms of action, such that distinctions between binding and gating modulations were possible. 6. Brief agonist exposure places limits on receptor activation and deactivation response kinetics. Consequently, receptor system responses may be used to characterize and functionally phenotype an excised patch receptor population. Furthermore, since synaptic exposure to transmitter is postulated to be similarly brief, IPSC kinetics may reflect a functional fingerprint of synaptic receptors.