Bawin and her coworkers have reported changes in binding of calcium after exposure of avian brain tissue to nonionizing electromagnetic radiation. Because calcium is intimately involved in the electrical activity of the brain, their results reveal a heretofore unrecognized potential for nonionizing radio-frequency radiation to affect biological function. We have verified and extended their findings. The forebrains of newly hatched chickens, separated at the midline to provide treatment-control pairs, were labeled in vitro with radioactive calcium. Samples of tissue were exposed for 20 minutes in a Crawford irradiation chamber to 147-MHz radiation, which was amplitude modulated sinusoidally at selected frequencies between 3 and 30 Hz. Power densities of incident radiation ranged between 0.5 and 2 mW cm−2. Compared with nonirradiated samples, a statistically significant increase in efflux of calcium ions (P < 0.01) was observed in irradiated samples at a modulation frequency of 16 Hz and at a power density of 0.75 mW cm−2. Our data confirm the existence of the frequency “window” reported by Bawin et al., as well as a narrow power-density “window” within which efflux of calcium ions is enhanced.