Drosophila Malpighian (renal) tubules are a powerful and adaptable model for studying in vitro epithelial transport mechanisms. Mutations in the white gene were the very first identified by Morgan. White is one of the most important genetic markers in modern Drosophila molecular biology; however, its function is surprisingly poorly understood. Previous studies have shown that white gene encodes a member of the G family [1] of ATP binding cassette (ABC) transporters, and is implicated in the transport of kynurenine, tryptophan and guanine. However, the function of white is surprisingly poorly understood considering it is one of the most important markers in modern Drosophila molecular biology. Cyclic guanosine 3′,5′-cyclic monophosphate (cGMP) is a signalling molecule involved in the regulation of a diverse range of tissues [2]. Cyclic GMP can be actively transported by various ABC transporters [3]. In a recent Affymetrix microarray experiment white was found to be 10.3 ± 1.4 (mean ± SEM, N=5) up-regulated in the Malpighian tubules [4] and differentially expressed when the tubules were incubated with cGMP. As the tubules are stimulated by exogenous cGMP, and are known to transport cyclic nucleotides, we hypothesised that white might be involved in cGMP transport. Using ³H labelled cGMP in a transport assay [5], wild-type tubules were shown to transport cGMP at a rate of 2.22 ± 0.24 fmol min^-1 (mean ± SEM, N=19), while in white mutant the rate of transport was 0.86 ± 0.09 fmol min^-1 (mean ± SEM, N=18), significantly (P<0.001) less cGMP than wild type tubules. A number of pharmaceutical competitors effect on cGMP transport were characterized, including glibenclamide, methotrexate, kynurenine, and tryptophan. Interestingly cAMP does not compete with cGMP transport, suggesting that the transport of these cyclic nucleotides is independent in Drosophila renal tubules. The white protein appears to be expressed at the basolateral membrane, using ICCs and a GFP-labelled overexpression construct. Further secretion assays demonstrate that even in the presence of cGMP transport inhibitors, or in white mutants, cGMP stimulates fluid secretion by tubules, suggesting an independent receptor-mediated action of cGMP, that does not require transepithelial transport.