It has been reported that exposure to electromagnetic fields influences intracellular signal transduction. We studied the effects of exposure to a time-varying 1.5T magnetic field on membrane properties, membrane cation transport and intracellular Ca 2+ mobilization in relation to signals. We also studied the mechanism of the effect of exposure to the magnetic field on intracellular Ca 2+ release from Ca 2+ stores in adrenal chromaffin cells.We measured the physiological functions of ER, actin protein, and mitochondria with respect to a neurotransmitter-induced increase in Ca 2+ in chromaffin cells exposed to the time-varying 1.5T magnetic field for 2h.Exposure to the magnetic field significantly reduced the increase in [Ca 2+ ]i. The exposure depolarized the mitochondria membrane and lowered oxygen uptake, but did not reduce the intracellular ATP content. Magnetic field-exposure caused a morphological change in intracellular F-actin. F-actin in exposed cells seemed to be less dense than in control cells, but the decrease was smaller than that in cytochalasin D-treated cells. The increase in G-actin (i.e., the decrease in F-actin) due to exposure was recovered by jasplakinolide, but inhibition of Ca 2+ release by the exposure was unaffected.These results suggest that the magnetic field-exposure influenced both the ER and mitochondria, but the inhibition of Ca 2+ release from ER was not due to mitochondria inhibition. The effect of eddy currents induced in the culture medium may indirectly influence intracellular actin and suppress the transient increase in [Ca 2+ ]i.