Previous reports have attributed increased carotid blood flow to lung ventilation during CPR. This study was undertaken to determine whether PEEP (20 torr airway pressure) or increased frequency of ventilation (3:1) improved arterial pressure, flow, and blood gases. Thirty-three domestic pigs were studied using three protocols (standard 5:1 CPR, 5:1 CPR plus PEEP, 3:1 CPR) distributed such that comparisons could be made between groups as well as within the same pig. For intrapig comparisons, PO2 was significantly improve (p less than 0.05) by PEEP (delta PO2 = 9.7 +/- 13.0 torr). PCO2 was significantly less (p less than 0.005) for 3:1 compared to 5:1 (delta PCO2 = -4.7 +/- 2.1 torr). For group comparisons, PO2 was 55.5 +/- 12.9 torr without and 70.1 +/- 16.3 torr with PEEP (p less than 0.025). For 3:1, PO2 was 66.3 +/- 11.6 torr that was greater (p less than 0.10) than for 5:1. When ventilation was temporarily halted, phasic changes in flow with ventilation were replaced by nearly constant flow approximately equal to maximal flow when ventilation was provided. PEEP and more frequent ventilation improved blood oxygenation but at the expense of carotid blood flow.