The familiality of infant sleeping heart rate (HR) and cardiac-related autonomic activity, as indexed by spectral analysis of HR variability (HRV) and response to postural change, was investigated in 322 5-month-old twin pairs. The postural change elicited only minor changes in cardiovascular activity. As a result, analyses focused on supine cardiovascular activity. Multivariate genetic modeling indicated that individual differences in sleeping HR and high frequency (HF) HRV were determined by unique environmental and distinct additive genetic factors. These variables, along with low frequency (LF) HRV, were also affected by overlapping familial environmental influences. Familial influences in HF HRV were more pronounced for baby girls than boys. Estimates of relative LF and HF HRV were determined by common (familial) and unique environmental factors; familial influences on these estimates did not overlap with familial influences on sleeping HR.