Instantaneous heart rate, indices of long-term and short-term heart-rate variability (HRV), and transcutaneous O2(PtcO2) and CO2(PtcCO2) tensions were recorded simultaneously on 164 occasions in 16 neonates. There was significant inverse correlation between PtcCO2and both HRV indices, while no linear correlation was detected between HRV and PtcO2. The heart rate was positively related to PtcO2and inversely correlated with PO2. It is suggested that increasing PCO2decreases medullary pH, thus increasing heart rate and decreasing HRV. We conclude that each of these monitoring variables is unique: the transcutaneous measurements display the efficiency of respiration, whereas the heart-rate patterns reflect the dynamic condition of the autonomic nervous system.