The electrophysiological signal elicited by a synthetic mosquito repellent, N, N-diethyl-3-methylbenzamide(DEET), on the antennae of Culex pipiens pallens suggested that a gas chromatography coupled with an electroantennographic detector (GC-EAD) could be applied for the screening of naturally occurring mosquito repellents. Using this approach we have identified geranial and neral as the active components of lemengrass. Cymbopogon citratus, a plant with known repellent activity. In a glass membrane feeder bioassay these EAD-active terpene compounds at 0.1mg/$ extrm{cm}^2$ dosage had repellent effect against C. p. pallens, Culex pipiens quinquefasciatus and Aedes albopictus. On the other hand, DEET (at lower dosages) showed repellence comparable to that of geranial. In addition, the synthetic repellent had a longer residual effect than the natural compounds. Only at a higher dosage (0.2mg/$ extrm{cm}^2$) did the activity of geranial persisted for 3h after the application.