The development of variability in heart rate (HR) due to respiration (sinus arrhythmia; SA) has been examined in normal infants from birth through the first 6 months of life. Two aspects of HR variation were examined: the absolute variation at the median respiratory frequency, or extent of sinus arrhythmia (XSA), and the degree to which HR follows respiration regardless of the absolute amount of variation, or coherence of sinus arrhythmia (CSA). Extent of sinus arrhythmia tended to be highest in quiet sleep (QS), lower in active or REM sleep (AS), and lowest in waking (AW), especially after 2 months of age. Extent declined at 1 month of age in QS, but rose over the first 6-month period in all states. During this same period, CSA was also highest in QS, lower in AS, and lowest in AW. Coherence in QS also declined at 1 month and rose between 1 and 6 months; however, no age effects were found in other states. Heart rate was negatively correlated with XSA, but less so with CSA. Sleep state appears to have a significant effect on cardiorespiratory coupling, and this coupling undergoes dramatic changes at 1 month in QS.