In blowfly salivary glands secretion of the KCl-rich saliva is stimulated by the neurohormone serotonin (5-HT). We studied intracellular pH changes caused by 5-HT.Intracellular pH was measured microfluorometrically using the pH-sensitive fluorescent dye BCECF.We found:1. Resting pH_i:7.53 ± 0.34, 2. buffer capacity is very high and pH_i-dependent, 3. 5-HT induces a dose-dependent and reversible intracellular acidification, 4. direct stimulation of the V-ATPase with cAMP (10 mM),cAMPS-Sp & 8-CPT-cAMP, IBMX or forskolin also causes an intracellular acidification, 5. 5-HT-induced acidification is HCO₃-, Cl^-- & Na ⁺-dependent 6. 5-HT-induced acidification is blocked by carbonic anhydrase inhibitors acetazolamide and ethoxyzolamide. Under these conditions, 5-HT induces a reversible alkalinization, this alkalinization is reduced by V-ATPase inhibitor concanamycin A 7. Histochemical detection of carbonic anhydrase reveals a basal localization.We conclude: The 5-HT-induced acidification is caused by metabolic CO₂ production that masks the alkalinization produced by V-ATPase mediated outward proton pumping.(Supported by DFG: Wa 463/9-3)