Calcium signaling is an important mediator of neuropeptide-stimulated fluid transport by Drosophila Malpighian (renal) tubules. We demonstrate the first epithelial role, in vivo, for members of the TRP family of calcium channels. RT-PCR revealed expression of trp, trpl, and trpgamma in tubules. Use of antipeptide polyclonal antibodies for TRP, TRPL, and TRPgamma showed expression of all three channels in type 1 (principal) cells in the tubule main segment. Neuropeptide (CAP(2b))-stimulated fluid transport rates were significantly reduced in tubules from the trpl(302) mutant and the trpl;trp double mutant, trpl(302);trp(343). However, a trp null, trp(343), had no impact on stimulated fluid transport. Measurement of cytosolic calcium concentrations ([Ca(2+)](i)) in tubule principal cells using an aequorin transgene in trp and trpl mutants showed a reduction in calcium responses in trpl(302). Western blotting of tubule preparations from trp and trpl mutants revealed a correlation between TRPL levels and CAP(2b)-stimulated fluid transport and calcium signaling. Rescue of trpl(302) with a trpl transgene under heat-shock control resulted in a stimulated fluid transport phenotype that was indistinguishable from wild-type tubules. Furthermore, restoration of normal stimulated rates of fluid transport by rescue of trpl(302) was not compromised by introduction of the trp null, trp(343). Thus, in an epithelial context, TRPL is sufficient for wild-type responses. Finally, a scaffolding component of the TRPL/TRP-signaling complex, INAD, is not expressed in tubules, suggesting that inaD is not essential for TRPL/TRP function in Drosophila tubules.