The duration of repellency of 4 different mosquito repellents was determined against heavy field populations of Anopheles freeborni Aitken (88 bites per hr). Four volunteers participated in the study. A dose of 0.4 mg/cm2 of each repellent was applied to 1 of 4 sites on the forearms. The criterion for failure of a repellent was 5 bites per site. Ranges of protection time obtained during the 4 day study were as follow: Cyclohexamethylene carbamide (carbamide) 10 to 16+ hr, triethylene glycol monohexyl ether (SRI-6) 4-12 hr, 1-(butylsulfonyl) hexahydro-1H-azepine (sulfonamide) 4-10 hr, and N, N-diethyl-m-toluamide (m-Deet) less than 4 to 10 hr. The 3 experimental repellents provided equal or greater protection than m-Deet for each volunteer.