AbstractTwo independent laboratories have demonstrated that electromagnetic radiation at specific frequencies can cause a change in the efflux of calcium ions from brain tissue in vitro. In a local geomagnetic field (LGF) at a density of 38 microTesla (μT), 15‐ and 45‐Hz electromagnetic signals (40 Vp‐p/m in air) have been shown to induce a change in the efflux of calcium ions from the exposed tissues, whereas 1‐ and 30‐Hz signals do not. We now show that the effective 15‐Hz signal can be rendered ineffective when the LGF is reduced to 19 μT with Helmholtz coils. In addition, the ineffective 30‐Hz signal becomes effective when the LGF is changed to ±25.3 μT or to ± 76 μT. These results demonstrate that the net intensity of the LGF is an important variable. The results appear to describe a resonance‐like relationship in which the frequency of the electromagnetic field that can induce a change in efflux is proportional to a product of LGF density and an index, 2n + 1, where n = 0,1. These phenomenological findings may provide a basis for evaluating the apparent lack of reproducibility of biological effects caused by low‐intensity extremely‐low‐frequency (ELF) electromagnetic signals. In future investigations of this phenomenon, the LGF vector should be explicitly described. If the underlying mechanism involves a general property of tissue, then research conducted in the ambient electromagnetic environment (50/60 Hz) may be subjected to unnoticed and uncontrolled influences, dependin
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