We have performed continuous recording of cutaneous PO2 (cPO2) together with a polygraphic recording of sleep in 12 normal premature babies, 36-38 weeks of post-conceptional age. In all babies, cPO2 was significantly lower (77.2 +/- 23.2) and more variable in active sleep than in quiet sleep (81.8 +/- 24.6). This difference is significantly related to the more numerous apneas, to the asynchronism of thoracic and abdominal movements, and to the motor activity with important irregularities of ventilation observed during the stage of active sleep.