Daytime measurements were made of the relationship between sleep state, cardio-respiratory changes and oxygen (transcutaneous PO2) and carbon dioxide (mass spectrometer alveolar PCO2) tensions in eleven normal infants from birth to 6 months of age. Sequential records were made at 1 week, 1, 3 and 6 months. The babies were not restrained, slept in a cot, and were constantly observed. Heart rate was higher in active sleep than in quiet sleep at all ages and rose between 1 week and 1 month (P less than 0.01) in both quiet sleep and active sleep and then decreased at 3 and 6 months. Respiratory rate only changes with age in active sleep at 6 months, showing a decrease (P less than 0.05) but was higher in active sleep that in quiet sleep at 1 month (P less than 0.002) and 3 months (P less than 0.02). A fall in heart rate (less than 100 beats/min) occurred chiefly in association with apnoea. The incidence of apnoea (of greater than 3 s duration) varied widely between babies at the same age and in the same baby at different ages but was most common in the first 3 months of life. No evidence of obstructive apnoea was found and no apnoea of greater than 12 s duration was observed. Period breathing occurred in all babies at 1 month normally following sighs but was uncommon at 1 week and 3 months and absent at 6 months. A fall in transcutaneous PO2 did not precede but often resulted from an episode of periodic breathing.