Low-level electromagnetic fields (EMFs) have been used to treat various neurologic disorders. In the present study, we applied micro Gauss (microG) levels of EMFs either to the vagosympathetic nerve trunks, dissected in the neck, or across the chest in anesthetized dogs. Based on theoretical and empiric grounds, we compared EMFs (2.87 microG at 0.043 Hz) delivered to the vagosympathetic trunks in an experimental set (n = 5) with a sham control group (n = 6). Over a period of 2 to 3 hours, heart rate decreased after an initial 5-minute EMF exposure. The maximal heart rate changes in the experimental versus control groups was 29% versus 12% (P = 0.03). The voltage applied to the autonomic nerves required to induce atrioventricular (AV) conduction block decreased by 60% in the experimental group versus a 5% increase in the control group (P = 0.005). This effect also lasted 2 to 3 hours. Another EMF setting (amplitude 0.34 microG, frequency 2 kHz) applied for 5 minutes to the vagosympathetic trunks was associated with a significant increase in the occurrence of atrial premature depolarizations (APDs), atrial tachycardia (AT), and atrial fibrillation (AF) in response to autonomic nerve stimulation compared with control states before EMF exposure. No atrial arrhythmias could be induced after propranolol and atropine, even at the highest voltage used to stimulate the autonomic nervous input to the heart (n = 11). Only 2 dogs showed no response to this EMF application. In 3 dogs in whom atrial pacing (cycle length = 250 ms) and autonomic nerve stimulation induced AF, an EMF (2.87 microG at 0.043 Hz) delivered for 35 minutes across the chest suppressed AF for up to 3 to 4 hours, after which the same protocol again induced AF. We conclude that in these preliminary experiments, specific low-level EMFs alter heart rate, AV conduction, and heart rhythm. These effects were mediated through the autonomic nervous system inputs to the heart based on adjunctive effect of autonomic nerve stimulation and the inhibitory action of autonomic blockers.