The membrane responses to the glutamate receptor agonist kainate and the subsequent changes in intracellular Ca2+, H+ and Na+ concentration were measured in giant glial cells of the leech central nervous system using ion-selective microelectrodes and microfluorimetry of Fura-2. The membrane depolarization or membrane inward current of exposed neuropile glial cells in situ, evoked by 2-20 microM kainate, were reversibly blocked by 6-cyano-7-dinitroquinoxaline-2,3-dione (CNQX), (50-100 microM) and by Ni2+ (2 mM), but not by methoxyverapamil (D600, 500 microM), which blocked voltage-gated Ca2+ influx. Local iontophoretic application of kainate on to the somatic membrane of single neuropile glial cells in situ, resulted in CNQX-sensitive depolarization and rises in intraglial Ca2+ concentration similar to those observed with bath-application of the agonist, indicating the presence of non-N-methyl-D-aspartate-type (NMDA) glutamate receptors in the somatic membrane of these cells. In voltage-clamped glial cells bath-application of kainate (5-10 microM) evoked inward currents and an increase in the membrane conductance,. while the intracellular Ca2+ increased (up to 200 nM). This increase in Ca2+i was not affected by substitution of Na+ by Li+, indicating that it is not due to reversed Na+/Ca2 exchange following intracellular Na+ accumulation. The intracellular Na+ concentration increased (up to 40 mM), and the intracellular pH decreased (0.2-0.3 pH units) in voltage-clamped glial cells following bath application of kainate. All these changes of the concentration of intracellular cations were reversibly suppressed by CNQX and Ni2+. The results indicate that Ca2+, Na+ and H+ enter leech neuropile glial cells presumably through non-selective cation channels, activated by the non-NMDA glutamate receptor agonist kainate.