The oxygen saturation in 71 healthy paediatric patients (3.5 months to 16.7 years) was measured by pulse oximetry during transfer from the operating room to the recovery room. These measurements were recorded continuously while the patients breathed room air. Of the patients studied, 28.1 per cent exhibited significant arterial desaturation of less than or equal to 90 per cent. The corresponding PO2 for this saturation level is less than or equal to 58 mmHg. In only 45 per cent of these desaturated patients was the desaturation recognized clinically by the presence of cyanosis. Age, type of anaesthetic, the use or avoidance of narcotics, and the use of controlled or spontaneous respiration had no significant relationship to the incidence of desaturation. Since more than a one quarter of all patients studied desaturated significantly, and since cyanosis can be difficult to detect clinically during the transfer period, the use of supplemental oxygen during transfer should be considered by the anaesthetist at the end of every paediatric general anaesthetic.