Forty-two babies under 5 days of age were observed for prolonged periods in a laboratory with instruments which markedly facilitate detection of cardiac rate and respiratory changes. Many newborn infants often manifest remarkable cardiac slowing associated with such diverse activities as hiccups, yawning, and defecation. The nasogastric intubation of premature infants has, at times, been followed by abrupt bradycardia. Sinus arrhythmia is a frequent occurrence. These seemingly diverse observations have implications for developmental physiology and may be relevant to some pathologic conditions unique to this period of life.