The effects of electromagnetic fields (EMFs) on the differentiation of cardiomyocytes in embryoid bodies derived from pluripotent embryonic stem (ES) cells were investigated. A single direct current (DC) field pulse was applied to 4-day-old embryoid bodies. The electrical field induced a hyperpolarization of the anode-facing side of embryoid bodies and a depolarization at the cathode-facing side. Significant effects of a single electrical field pulse applied for 90 s on cardiomyocyte differentiation were achieved with field strengths of 250 and 500 V/m, which increased both the number of embryoid bodies differentiating beating foci of cardiomyocytes and the size of the beating foci. The 500-V/m electrical field increased intracellular reactive oxygen species (ROS), but not [Ca(2+)](i) and activated nuclear factor kappa B (NF-kappaB). A comparable increase in the number of beating embryoid bodies was achieved by an incubation for 1 h with H(2)O(2) (1-10 nM), indicating that the electrical field effect was transduced via the intracellular generation of ROS. Because the radical scavengers dehydroascorbate and pyrrolidinedithiocarbamate (APDC) and the NF-kappaB antagonist N-tosyl-L-phenylalanine chloromethyl ketone (TPCK) inhibited cardiac differentiation, we assume that ROS and NF-kappaB may play a role in early cardiac development.