Development of heart rate variation in three frequency ranges was examined during sleep-walking states in normal infants over the first 6 mo of life. Extent of all three types of heart rate variation decreased from 1 wk to 1 mo of age. Extent of respiratory sinus arrhythmia increased from 1 mo to 6 mo during all sleep-waking states, with the increase most pronounced during quiet sleep. Variation in two bands of lower frequencies showed increases in extent from 1 to 3 mo, then a slowing or reversal of the increase between 3 and 4 mo of age. During rapid eye movement sleep, the two types of lower frequency heart rate variation decreased in extent from 3 through 6 mo of age. These results suggest that alterations in autonomic control of heart rate occur at several time periods over the first 6 mo of life and that these alterations may have an effect only on particular types of heart rate variation and only during particular sleep-waking states. The diminution of all three types of heart rate variation at 1 mo may indicate a reduction in vagal tone at this age.