Previous studies show alterations in the dynamic patterns of cardiac rate in several "at-risk" populations, including apparently healthy infants who subsequently die of the sudden infant death syndrome. In the present study, we examined the maturation of cardiac rate dynamics in normal infants during sleep-waking states over the first 6 mo of life. Instantaneous changes in cardiac R-R intervals were examined in 12-h recordings of 24 normal full-term infants; each infant was recorded at 1 wk and at 1, 2, 3, 4, and 6 mo of age. Scatter plots, consisting of each cardiac R-R interval plotted as a function of the previous interval (Poincaré plots), were constructed for each sleep-waking state in each recording. Analyses of variance were performed on the dispersion of intervals after long and short R-R intervals. In neonates, Poincaré plots showed significantly more next-interval dispersion after a long R-R interval than after a short interval, a pattern similar to those observed in older infants and in healthy adults. However, between 1 wk and 1 mo of age, this pattern disappeared and returned gradually beginning at 2 mo of age. The scatter of points in Poincaré plots of infants 1 mo of age approached the patterns of at-risk populations, including infants who subsequently died of the sudden infant death syndrome. These patterns at 1 mo may be indicative of increased vulnerability in normal infants after the neonatal period.