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A frequent problem in the radiofrequency (RF) irradiation of experimental animals in health effects studies is the temporal variation of the specific absorption rate (SAR) with animal movement. An RF power controller that regulates the energy absorption rate has been designed for use with transmission line exposure systems that utilize the power difference method to monitor the SAR. The controller operates by altering the incident power to the exposure cell in order to compensate for the change in RF energy absorption rate that is due to animal motion. A circuit diagram is presented as well as experimental data under three conditions of exposure. The controller is effective in maintaining the mean value of energy absorption rate at the setpoint value even for the case of a highly active animal.