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Cardiac measures of heart period and high‐frequency heart period variability are increasingly employed as dependent variables in studies of social and emotional development in infancy and childhood. This study describes significant developmental increases in these measures in a longitudinal sample assessed at 4, 9, 14, 24, and 48 months of age. In addition, developmental changes in the characteristics of the heart period power spectra are described. These changes have implications for the quantification of high‐frequency heart period variability in infancy and childhood. First, shorter analysis epoch lengths may be used for younger infants. Second, the commonly used high‐frequency band for infants (0.24–1.04 Hz) appears to reach its practical limit at an age of around four years. Findings are discussed in relation to the design of developmental psychophysiological studies. © 2000 John Wiley & Sons, Inc …