Vector-borne pathogens develop in close association with specific tissues in their insect hosts. Efforts are being made to characterize insect genes that are expressed in tissues that have important roles in pathogen propagation. Successful transfection and expression of exogenous genes in terminally differentiated tissues of insects has previously proven difficult. Here we report a method that should allow the analysis of genes that are expressed in adult tissues and organs. Transient expression assays have been developed using the salivary glands of the mosquito, Aedes aegypti, which can now be used to analyze salivary gland-specific promoter sequences. A liposome-based transfection reagent was used to transfect cultured adult salivary glands with a DNA construct carrying the luciferase reporter gene under the control of the Drosophila melanogaster heat shock 70 promoter. Luciferase activity was detected in glands 18–20 hr post-transfection. This assay can now be used to determine the regulatory activity of other putative promoter sequences from salivary gland-specific genes. Alternatively, the assay may be used to study the effect of recombinant gene expression on parasite invasion and development. In addition, transient expression of gene constructs in embryos is shown to be a powerful tool for analyzing genes that are expressed at this stage of the mosquito life cycle.