SUMMARY Glial cells can be activated by neurotransmitters viametabotropic, G protein-coupled receptors. We have studied the effects of`global' G protein activation by GTP-γ-S on the membrane potential,membrane conductance, intracellular Ca2+ and Na+ of the giant glial cell in isolated ganglia of the leech Hirudo medicinalis. Uncaging GTP-γ-S (injected into a giant glial cell as caged compound) by moderate UV illumination hyperpolarized the membrane due to an increase in K+ conductance. Uncaging GTP-γ-S also evoked rises in cytosolic Ca2+ and Na+, both of which were suppressed after depleting the intracellular Ca2+ stores with cyclopiazonic acid (20 μmol l–1). Uncaging inositol-trisphosphate evoked a transient rise in cytosolic Ca2+ and Na+ but no change in membrane potential. Injection of the fast Ca2+ chelator BAPTA or depletion of intracellular Ca2+ stores did not suppress the membrane hyperpolarization induced by uncaging GTP-γ-S. Our results suggest that global activation of G proteins in the leech giant glial cell results in a rise of Ca2+-independent membrane K+conductance, a rise of cytosolic Ca2+, due to release from intracellular stores, and a rise of cytosolic Na+, presumably due to increased Na+/Ca2+ exchange.