The objective of the present study was to investigate the emergence of circadian patterns in respiratory rates. Twenty-five neurologically normal infants were monitored for 12-hours during the first week of life and at one, two, three, four and six months of age. Each minute of the recordings of EEG, eye movements, somatic activity, respiration, ECG and temperature was coded into Active sleep (AS), Quiet sleep (QS), Indeterminate (IN) and Awake (AW). The night was divided into four equal intervals and a computer program calculated the mean respiratory rate for each interval as a function of state and age. During the first week of life group means of respiratory rates increased linearly with each interval in all states. A circadian pattern with lowered respiratory rates between 10:00 p.m. and 1:00 a.m. emerged at different ages in different sleep states. This pattern was first seen at one month of age in AS and IN, and at three months in QS. It was also observed in AW at three months of age, suggesting that the circadian rhythm is not solely an expression of the sleep wakefulness cycle. Feeding patterns contributed to but did not explain these observations.