AbstractWe have previously shown that 16‐Hz, sinusoidal electromagnetic fields can cause enhanced efflux of calcium ions from chick brain tissue, in vitro, in two intensity regions centered on 6 and 40 Vp‐p/m. Alternatively, 1‐Hz and 30‐Hz fields at 40 Vp‐p/m did not cause enhanced efflux. We now demonstrate that although there is no enhanced efflux associated with a 42‐Hz field at 30, 40, 50, or 60 Vp‐p/m, a 45‐Hz field causes enhanced efflux in an intensity range around 40 Vp‐p/m that is essentially identical to the response observed for 16‐Hz fields. Fields at 50 Hz induce enhanced efflux in a narrower intensity region between 45 and 50 Vp‐p/m, while radiofrequency carrier waves, amplitude modulated at 50 Hz, also display enhanced efflux over a narrow power density range. Electromagnetic fields at 60 Hz cause enhanced efflux only at 35 and 40 Vp‐p/m, intensities slightly lower than those that are effective at 50 Hz. Finally, exposures over a series of frequencies at 42.5 Vp‐p/m reveal two frequency regions that elicit enhanced efflux—one centered on 15 Hz, the other