1. We have studied extracellular pH (pHe) and intracellular pH (pHi) changes evoked by repetitive electrical side nerve stimulation (20 Hz, 1 min) in segmental ganglia of the leech Hirudo medicinalis using double-barreled, pH-sensitive microelectrodes to elucidate the involvement of neurotransmitters, of carbonic anhydrase, and of active acid/base transport on the extracellular H+ homeostasis. In saline buffered with 5% CO2-24 mM HCO3-, the stimulation induced a small and brief alkalinization followed by an acidification in the extracellular spaces (ECS), whereas neurons acidified and glial cells alkalinized (see previous paper). 2. Blocking synaptic transmitter release by superfusion with 20 mM Mg2+ saline (CO2/HCO3(-)-free) led to a reversible reduction of both activity-induced pHe changes by approximately 90% and to a complete suppression of the intracellular acidification of neurons. After application of the glutamate/kainate receptor blocker 6-cyano-7-dinitroquinozaline-2,3-dione (CNQX, 50 microM) to CO2/HCO3(-)-free saline, the stimulus-evoked pHe changes were reversibly reduced. The gamma-aminobutyric acid-A (GABAA) receptor antagonist picrotoxin (50 microM) led to an amplification of the extracellular alkalinization in the presence of CO2/HCO3-. Bath application of the excitatory transmitter agonists carbachol or kainate to CO2/HCO3(-)-free saline induced biphasic alkaline-acid transients in the ECS; the inhibitory transmitters GABA and serotonin had no detectable effects on the pHe (saline buffered with CO2/HCO3-).(ABSTRACT TRUNCATED AT 250 WORDS)