A new test system was developed to assess the intrinsic repellency of chemical compounds and formulations to mosquitoes. The system incorporates an In Vitro blood-feeding device which allows unrestricted "free choice" feeding on various repellent-treated membrane surfaces, thereby providing comparative feeding data on a quantitative basis. Three configurations of the blood-feeding device permit the use of a variety of experimental designs. Examples of experiments employing factorial and dose-response designs are presented. The In vitro repellent test system was proved in more than 250 repellent tests utilizing a variety of chemicals and mosquito species and strains.