The effects of duration of radiofrequency (RF) radiation on rats were investigated. Pregnant Sprague-Dawley-rats were irradiated on day 9 of pregnancy in a RF near field synthesizer operated at 27.12 megaHertz in a dominant magnetic field mode. Rats were either sham exposed or exposed to magnetic fields at 55 Amps per meter or electric fields at 300 Volts per meter. In one group, the treatment was terminated when rat colonic temperature reached 41 degrees-C; in another group 41 degrees was maintained for 2 hours. Treatment was terminated at 42 degrees in the third group, and maintained for 15 minutes at 42 degrees in the fourth group. On day 20, the uterine horns were examined for number of implantations, live fetuses, and dead or resorbed conceptuses. The percentage of visceral abnormalities per litter, major skeletal abnormalities per litter, and minor skeletal variations per litter were higher in the 2 hour group at 41 degrees than in the group that was immediately terminated at 41 degrees. Mean fetal weight per litter was lower in the longer exposed groups. The frequency of malformations was also greater in the 15 minute exposure group at 42 degrees than in the group in which exposure was terminated at 42 degrees. Malformations included micrognathia, agnathia, microtia, anotia, exencephaly, encephalocoele, and facial aplasia. The authors conclude that the teratogenic and embryotoxic effects of RF induced hyperthermia are related to both temperature of the dam during exposure and length of time the dam's temperature remains elevated. The temperature threshold for malformation appears to decrease with increased exposure.