Electrocardiograms were recorded by needle electrodes in 137 of 166 rabbit fetuses at 10½ to 31 days. Invariably, electrocardiograms were obtainable from the 12th day on, which in the rabbit is an early stage of development. The earliest electrocardiogram was obtained at 10 days, 22 hours, corresponding to about five weeks in human development. Sinus rhythm was evident from the start. The wave form consists of a pair of complexes 50 to 200 msec apart. The first complex is quite variable but appears to be atrial in origin; the second is surprisingly constant and appears to be ventricular.