A system was developed for exposing a sample of cells in a culture medium for short periods to broadband electromagnetic energy under controlled conditions of field and temperature. The system can produce a reasonably uniform electric field of strengths to 100 V/cm in a 5-ml sample; the system has a continuously controllable input power to 600 W and a frequency range from dc to 100 MHz and beyond. Control of the sample's temperature over a wide range is achieved through a high-rate, circulating-liquid heat exchanger. The cooling capacity is sufficient to maintain one of a wide range of steady-state temperatures during exposure of the sample to a CW field of strength in excess of 20 V/cm. Accurate monitoring of the sample's temperature at frequencies below 100 MHz is achieved during exposures through on-line measurement of feedline impedance. Nonperturbing thermal measurement of sample can be made at all frequencies by a Teflon-insulated sensor of special design.