We have tested the effect of dextran (40 kDa, 5%) on miniature IPSCs (mIPSCs) recorded in layer V cortical pyramidal cells. This compound increases the amplitude of mIPSCs at room and physiological temperatures by 15%, leaving their duration unaffected at room temperature and slightly increased at physiological temperature. The amplitude increase is attributable to an increase in the number of receptors bound by GABA during synaptic transmission, as shown by the occlusion between the effects of dextran and zolpidem on mIPSC amplitude at room temperature. As dextran presumably enhances the concentration and dwell time of GABA in the synaptic cleft, these results demonstrate that the postsynaptic GABAA receptors are not saturated at room and physiological temperatures.