Three biological assay procedures for repellents are currently documented in the literature: 1) ASTM E951-94, Laboratory testing of non-commercial repellent formulations on the skin. 2) ASTM E939-94, Field testing topical applications of compounds as repellents for medically important and pest arthropods. 1. Mosquitoes. 3) WHO/CTD/WHOPES/IC/96.1, Report of WHOPES informal consultation on the evaluation and testing of insecticides. One public draft set of repellent-testing guidelines is available on the internet: 4) USEPA OPPTS 810.3700, Product performance test guidelines. Insect repellents for human skin and outdoor premises. In practice, the outcome of a repellent bioassay using any of these procedures is affected by the absorption, penetration, and chemical modification of repellent on skin and by evaporation, abrasion, and perspiration. Other abiotic factors that influence mosquito responses to repellent stimuli are light, temperature, humidity, repellent dose, exposure time, and test-cage shape and size. Biotic variables in repellent bioassays are larval nutrition, carbohydrate availability for adult mosquitoes, age and parity of females, and differences in the innate attraction/ repellency of test subjects. Geographic location and seasonal and diel activity cycles in mosquitoes determine when and where repellents can be tested in the field. Critical knowledge of these sources of variation can be converted to improved precision and accuracy in repellent bioassays and the resulting information used to efficiently select new repellent compounds for toxicological evaluation and field testing.