The heart rate (HR), heart rate variation (HRV) and the power spectrum of the HR were studied in 1 to 10-week-old neonatal lambs. The average HR was highest and the indices of the HRV were lowest at the age of 1 to 3 weeks. However, there was a decreasing trend in the overall HRV with advancing age when it was normalized in relation to the average HR. The absolute beat-to-beat HRV increased after the age of 3 weeks; it remained the same when normalized in relation to the HR. The overall HRV was highest during waking and active sleep; it was lowest during quiet sleep. The beat-to-beat HRV was lower during quiet sleep than during active sleep. Three major peaks of HRV power were detected in the spectra of all lambs. During active sleep the HRV power increased at all frequencies ranging from 0 to 1.0 Hz. During waking the low-frequency HRV power was primarily increased. The high-frequency HRV was pronounced during quiet sleep and the state of sleepy waking, especially at the age of 4 to 6 weeks. Our study indicates that developmental factors and the sleep state have to be considered when HR patterns are experimentally investigated. Many similarities between the HR control of neonatal lambs and that of human neonates make it feasible to study the mechanisms of clinical HRV patterns by lamb experiments.