Direct current-generated EM fields modulated evoked potentials recorded from hippocampal slices. Both the synaptic efficiency represented by the slope of EPSP and the number of activated pyramidal cells represented by the population spike were modified by EM fields. While the weak EM fields (2-3 mT) exerted a biphasic effect observed as a transient small depression followed by a long-lasting amplification of the potentials, stronger EM fields (8-10 mT) depressed these potentials. This depression could be partially reversed by increasing the strength of the stimulation. It is suggested that EM-induced fluctuations in the intracellular Ca2+ concentration are responsible for the observed effect.