Three modes of ventilation during the transport of 30 ventilator-dependent patients were compared using blood gas variables. Ten were ventilated with a manually operated, ventilation bag (group C) and ten with a tidal volume meter at the exhalation valve of the ventilation bag (group V). Another ten patients (group O) were ventilated with a portable ventilator set to the minute volume (VE) given in ICU. VE was measured by volumetry as described above. Blood gases were analyzed in the ICU before and at the end of transport. In group C, significant decreases occurred in arterial (p less than .01) and central venous (p less than .05) PCO2, as well as in central venous PO2 (p less than .01). Arterial (p less than .05) and central venous (p less than .01) pH increased. Group V showed no statistically significant changes. In group O, arterial (p less than .01) and central venous (p less than .05) PCO2 decreased whereas arterial (p less than .01) and central venous (p less than .05) pH increased. We conclude that VE should be monitored during transport of ventilated patients.