This study was carried out on 57 normal infants: 22 full-term newborns, examined in the hospital laboratory, and 35 2-18-wk-old infants, examined in two resident nurseries. Polygraphic records, including 1-3 complete sleep cycles, were performed during the morning. The tracings were analyzed by 20-sec epochs. Three to 10% of active sleep states (AS) and 0.8-4% of quiet sleep states (QS) included greater than or equal to 3 sec respiratory pauses. There were minimal, non-significant differences between respiratory frequencies (RF) in total and in no-pause tracings. Our results confirmed that RF was higher in AS in all ages, when compared with QS (P less than 0.02). During the transition (TS) from one to another well-defined sleep state, the respiratory rate showed an intermediate level (AS greater than TS greater than QS): the transition from AS to QS showed progressive slowing of RF, while the transition from QS to AS occurred abruptly, with sudden acceleration of RF. There was a significant slowing of RF during the couse of QS, while the RF in AS was more variable without significant differences between the beginning, the middle and the end of AS state. In this material RF was higher in 2-5-wk and 6-10-wk age groups, compared to newborns and to 11-18-wk-old infants. At all ages, there was a high degree of correlation (P less than 0.01) between RF found in different sleep states for given individuals: some infants breathed more rapidly and other more slowly in all sleep states. A review of the literature showed that the differences between normal RF.