This study compares continuously measured arterial oxygen tension (PaO2) in normal unanesthetized patients after four maximally deep breaths of 100% O2 within 30 seconds, to (PaO2) values observed after 5 minutes of spontaneously breathing 100% O2 at normal rates and tidal volumes. Mean (PaO2) after 5 minutes of O2 breathing was 350 +/- 36 (SD) torr, not significantly higher (p greater than 0.05) than after four breaths of O2 (339 +/0 34 torr). Mean arterial oxygen content was 19.6 +/- 2.1 vol % in both periods. For purposes of increasing arterial oxygenation before anesthetic induction the four-breath, 30-second technique and 5 minutes of oxygen breathing are equally effective.