Postnatal adaptation should be associated with changes in cardiac rhythmic behavior. To examine the development of heart rate variability, instantaneous heart rate (IHR) and the corresponding breathing signals of 16 healthy infants were analyzed. This was pursued by use of fast Fourier transformation beginning with the 1st day until the 6th mo of life. Power in the low-frequency range (LF, 0.02-0.2 Hz) and high-frequency range (HF, 0.2-1.5 Hz), total power (TP), the quotient LF/HF, and the frequency of the peak in LF and HF (LFF and HFF, respectively) were derived from the IHR spectrum. The peak frequency in HF (RF) was detected in the respiratory spectrum. Power and frequency of IHR rhythms undergo a marked development. TP, LF, and HF are lowest from the end of the 1st mo until the 2nd mo. LF predominates over HF, with LF/HF reaching its peak during 1- to 2-mo period. HF, recording respiratory related rhythms is negatively correlated with the breathing rate (BR). HFF and RF both show an increasing tendency during the 1st mo followed by a decrease down to the 6th mo. However, HFF is lower than RF if BR is high, mainly during the first 2 mo. The distinct changes in BR and its important influence on the IHR spectrum underscore the importance of monitoring respiration as a further measure in the diagnosis of infants. LFF is on average between 0.075 and 0.095 Hz, exhibiting an irregular course with minimum at the 10th, 21st-28th, and 90th day being apparent. The developmental pattern of LFF may by interpreted in terms of the maturation of the nervous system involved in the generation of circulatory rhythms.