RNA interference (RNAi) is an important technique for assessing gene function. However, RNAi varies in its effectiveness, depending on the target organism and gene. We tested RNAi approaches to reduce expression of salivary apyrase in Aedes aegypti. Constructs were made containing sense-antisense inverted repeats, which direct synthesis of a single mRNA molecule that folds to form 400 bp of double-stranded RNA (dsRNA) corresponding to the 5'-end of the apyrase mRNA. E. coli containing this plasmid was induced with IPTG and fed to A. aegypti larvae, resulting in a 50% reduction in apyrase titers. Controls fed E. coli containing a sense-sense construct that does not produce dsRNA, had no reduction in apyrase titers. Salivary anticoagulant titers were not inhibited in either group of mosquitoes, indicating that the RNAi effect was specific for apyrase. The sense-antisense construct was inserted into Sindbis virus, which was used to infect freshly emerged female mosquitoes. Apyrase titers were reduced by 85% by day 9 post-innoculation, and by 97% at day 15. Virus expressing the control construct also reduced apyrase titers, although more slowly. Again, anticoagulant activities were not inhibited in either group. Feeding dsRNA in E. coli to larvae is a suitable technique for associating specific cDNAs with biological activities that can be assessed using dose-response experiments. Infection with Sindbis virus expressing dsRNA constructs may produce sufficient knockdown to directly assess phenotypes.