One of the most interesting and controversial papers on the biological effects of nonionizing radiation was published by Bawin, Kaczmarek and Adey in 1975. They found a 147 MHz carrier wave could elicit and enhance efflux of calcium ions from chick brain tissue only when amplitude modulated at certain sub-ELF frequencies. In addition to being one of the few U. S. reports at that time which described a biological response to an exposure at a power below 10 mW/cm. sq., the results demonstrated a modulation frequency-specific response with a maximum effect at 16 Hz. This response was particularly significant because the effective modulating frequencies were within the range of frequencies found in the electroencephalogram (EEG) of the intact animal. An important feature of the research was the relatively simple biological procedure: halves of chick brains were labeled with a radioisotope (45Ca++), exposed to RF fields for a short time, and the amount of 45Ca++ released into the medium during irradiation was measured. In 1979, the authors reported their success in replicating the essential characteristics of the frequency response curve. However, success was achieved only after exploring a range of power densities at 147 MHz carrier frequency, 16 Hz amplitude modulation. The result demonstrated the existence of a power density window at 0.83 mW/cm. sq. in that no enhanced calcium efflux was found at either higher or lower power densities. Subsequent to this work, we examined the effect of 9 Hz modulation on the power density response and found that the location of the window was unchanged.