In previous experiments changes were found in calcium-ion efflux from chick-brain tissue that had been exposed in vitro to 147-MHz radiation across a specific range of power densities when the field was amplitude modulated at 16 Hz. In the present study, 50-MHz radiation, similarly modulated as a sinusoid, was found to produce changes in calcium-ion efflux from chick brains exposed in vitro in a Crawford cell. Exposure conditions were optimized to broaden any power-density window and to enhance the opportunity to detect changes in the calcium-ion efflux. The results of a power-density series demonstrated two effective ranges: One spanning a range from 1.44 to 1.67 MH/sq. cm, and the other including 3.64 mW/sq. cm, which were bracketed by no-effect results at 0.72, 2.17, and 4.32 mW/sq. cm. Peaks of positive findings are associated with near-identical rates of energy absorption: 1.4 microwatts/g at 147 MHz, and 1.3 microwatts/g at 50 MHz, which indicates that the enhanced-efflux phenomenon is more dependent on the intensity of fields in the brain that on the power density of incident radiation. In addition, the phenomenon appears to occur at multiples of some, as yet unknown, rate of radiofrequency (RF) energy absorption. Because of the extremely small increments of temperature associated with positive findings (<4 X 0.0001 degrees C), and the existence of more than one productive absorption rate, a solely thermal explanation appears extremely unlikely.