In the present study, isolated midguts of larval Aedes aegypti L. (Diptera: Culicidae) were mounted on perfusion pipettes and bathed in high buffer mosquito saline. With low buffer perfusion saline, containing m-cresol purple, transepithelial voltage was monitored and luminal alkalinization became visible through color changes of m-cresol purple after perfusion stop. Lumen negative voltage and alkalinization depended on metabolic energy and were stimulated in the presence of serotonin (0.2 µmol l^-1). In some experiments a pH microelectrode in the lumen recorded pH values up to 10 within minutes after perfusion stop. The V-ATPase inhibitor concanamycin (50 µmol l^-1) on the hemolymph side almost abolished V_te and inhibited luminal alkalinization. The carbonic anhydrase inhibitor, methazolamide (50 µmol l^-1), on either the luminal or hemolymph-side, or the inhibitor of anion transport, DIDS (1 mmol l^-1) on the luminal side, had no effect on V_te or alkalinization. Cl^- substitution in the lumen or on both sides of the tissue affected V_te, but the color change of m-cresol purple was unchanged from control conditions. Hemolymph-side Na substitution or addition of the Na /H exchange inhibitor, amiloride (200 µmol l^-1), reduced V_te and luminal alkalinization. Luminal amiloride (200 µmol l^-1) was without effects on V_te or alkalinization. High K (60 mmol l^-1) in the lumen reduced V_te without affecting alkalinization. These results indicate that strong luminal alkalinization in isolated and perfused anterior midgut of larval A. aegypti depends on basolateral V-ATPase, but is apparently independent of carbonic anhydrase, apical Cl^-/HCO₃^- exchange or apical K /2H antiport.