Cardiac -rate changes with abdominal air-stream stimulation were studied in 14 normal infants during the newborn period and the subsequent 5 mo. of life. By 2 1/2 mo. the response pattern had changed significantly. This was characterized by a faster reaction, attenuated initial rise in rate, and generally a greater return to below pre stimulus levels. In the older infants the overall heart rates were higher than in their respective newborn periods and during crying often achieved levels of 220-230 beats/-min. Response measures at 2 1/2 and 5 mo. of age were often significantly correlated, demonstrating increasing stability after the newborn period. In the light of animal studies indicating early maturational changes in autonomic nervous-system function, it is hypothesized that significant changes in cardiac control mechanisms occur in the 1st months of life in the human infant.