The larval midgut epithelium of Drosophila is a useful model system for acid and base transport (Shanbhag &Tripathi 2009a). The posterior part secretes base into the lumen, driven primarily by a voltage-dependent, electrogenic H⁺ V-ATPase that extrudes H⁺ across the basal membrane into the haemolymph (Shanbhag &Tripathi 2005;Shanbhag &Tripathi 2009a, b). Pharmacological tools available for analysis of transporters in insect systems are limited. We have therefore sought to identify other primary or secondary transport pathways with ion-substitution or blockers. Posterior midgut segments were perfused and superfused for measurement of transepithelial electrical parameters (Shanbhag & Tripathi,2005) including intracellular ionic composition measured with liquid ion-exchange microelectrodes, Reduction of bath K⁺ (isoosmotically exchanged with Na⁺) from 5 to 0.5mM, reduced intracellular K⁺ from 19.6 ±5.8 to 13.4 ±4.9 mM (n=3, p<0.02), increased intracellular Na⁺ from 93.3 ±23.3 to 113 ±25mM (n=3,p<0.01) and decreased intracellular pH from 7.23 ±0.17 to 6.91 ±0.14 (n=10, p<0.01). Ba²⁺ (5mM) added to the bath reduced intracellular K⁺ from 26.9 ±10.6 to 14.0 ±2.6 mM (n=3, p<0.05), K⁺ removal or Ba²⁺ (5mM) addition at the luminal side was without significant effect on these intracellular ions. We conclude that there is a Na-K exchanger in the basal membrane, in parallel with K-channels and the H⁺ V-ATPase driving net salt absorption and net base secretion in this segment. Shanbhag, S., & Tripathi, S. (2005). J Membrane Biol 206, 61-72. Shanbhag, S. & Tripathi, S. (2009a). J. Exp. Biol. 212, 1731-1744. Shanbhag, S. and Tripathi, S. (2009b). Proc. Physiol. Soc. 16, C16. Where applicable, experiments conform with the Physiological Society ethical requirements