SUMMARY Diuretic hormone 44 (DH) is a bioactive neuropeptide that mediates osmotic balance in a wide variety of insects through increases in cAMP. It is structurally similar to mammalian corticotrophin releasing factor (CRF)peptides. In the moth Manduca and the cricket Acheta ,functional studies have shown that its cognate receptor (DH-R) is related to the mammalian CRF receptor. The Drosophila genome contains two genes( CG8422 and CG1237 0) orthologous to Manduca and Acheta DH-Rs. Here, we present multiple lines of evidence to support the hypothesis that the orphan CG8422 G-protein-coupled receptor is a functional DH-R. When expressed in mammalian cells, CG8422 conferred selective sensitivity to DH, as indicated by translocation of aβ-arrestin-2-GFP reporter from the cytoplasm to the cell membrane. Consistent with its in vivo activities in other insects, DH activation of CG8422 elicited increases in a cAMP reporter system( CRE-luciferase ), with an EC 50 of 1.7 nmol l -1 . CG8422 activation by DH also led to increases in intracellular calcium but at substantially higher doses (EC 50 ∼300 nmol l -1 ). By microarray analysis, the CG8422 transcript was detectable in Drosophila head mRNA of different genotypes and under different environmental conditions. The identification of a Drosophila receptor for the DH neuropeptide provides a basis for genetic analysis of this critical factor's roles in maintaining physiological homeostasis.