The possibility that venous blood could be withdrawn through a radial artery cannula when venous pressure is high was examined in 30 patients during cardiopulmonary bypass for coronary artery bypass operation. Progressive desaturation of radial artery blood occurred when venous pressure was equal to arterial pressure. Desaturation not only increased with duration of venous obstruction but also occurred immediately if the arterial tree was emptied after occlusion. We believe that these observations result from the transcapillary aspiration of deep venous blood and may produce a potential source of error in measuring arterial oxygen tension during cardiopulmonary resuscitation, cardiac tamponade, and profound right heart failure, as well as cardiopulmonary bypass.