The influence of sleep state on hypoxia-induced periodic breathing was evaluated in full-term babies studied during the first week of life. Following exposure to 15% O2, periodic breathing occurred in 7 of 42 babies during quiet sleep, and 5 of 20 during active sleep (p less than 0.05). The median levels of transcutaneous pO2 at which periodic breathing occurred during hypoxia, and was abolished on reoxygenation were similar in each sleep state. The median duration of periodic apnoea was significantly greater in quiet than active sleep. In 7 babies the occurrence of periodic breathing during hypoxia was accompanied by a change in sleep state. This was probably coincidental in 3 cases, but in the other 4 babies hypoxia may have induced both the change in state and breathing pattern.