As part of a long-range investigation of the cardiopulmonary response in monkeys exposed to toxic materials, normal electrocardiograms of 16 male cynomolgus monkeys (Macaca fascicularis), 3 to 5 years old, were studied. With the animals restrained in a sitting position, electrocardiograms were measured and were found to be similar to those of other subhuman primates. Standard and augmented limb leads (I, II, III, aVR, aVL, aVF) were obtained with a photographic recorder. Normal parameters were also determined, including PR and QT intervals, P-wave, QRS, and T-wave amplitudes, P-wave and QRS wave widths. Tracing was recorded during an interval when muscle artifacts from body tremor or movement were minimal. P-wave was found to be upright in leads I, II, III, and aVF, inverted in aVR, and frequently inverted or flat in aVL. P- wave axis averaged plus 71 degrees with a range of plus 50 to plus 90 degrees. P-wave width averaged 31 to 41msec. Atrioventricular conduction time averaged 80msec; ventricular systole lasted 200msec on the average. Tendency existed for shorter PR and QT intervals to occur at higher heart-rates, and longer intervals to occur at lower heart-rates. Mean heart-rate was 179 beats per minute, with a range of 135 to 215 beats per minute. It is concluded that lower voltages than have been previously reported for similar size monkeys are characteristic of Macaca fascicularis.