It is becoming increasingly evident that activation of ionotropic glutamate receptors leads to significant changes in cytosolic [K+] ([K+]c), a major determinant of the plasma membrane (PM) potential, Em. Since Em affects fluxes of key cations, such as Ca2+, it is important to precisely quantify [K+]c and Em in neurons exposed to glutamate receptor agonists. Here we studied the relationships between [K+]c and Em in primary cultures of cerebellar granule cells, and found that kainate elicits a rapid drop in [K+]c below 10 mM. Using patch electrodes containing 10 or 150 mM K+, we determined that kainate depolarizes the PM to -2 or -28 mV, respectively. Therefore, the actual PM depolarization elicited by kainate is much larger than that routinely measured with K+-rich electrodes.