Background. Relative to the nonbypass state, cardiopulmonary bypass may decrease whole-body oxygen (O-2) delivery. We predicted that during cardiopulmonary bypass, a hierarchy of regional blood now and O-2 delivery could be characterized. Methods. In 8 46.5 +/- 1.2-kg pigs, fluorescent microspheres were used to determine blood now and O-2 delivery to five organ beds before and during 37 degreesC cardiopulmonary bypass at four randomized bypass flows (1.4, 1.7, 2.0, and 2.3 L/min/m(2)). At completion, 18 tissue samples were obtained from the cerebral cortex (n = 4), renal cortex (n = 2), renal medulla (n = 2), pancreas (n = 3), small bowel (n = 3), and limb muscle (n = 4) for regional blood now determination. Results. At conventional cardiopulmonary bypass flow (2.3 L/min/m(2)), whole-body O-2 delivery was reduced by 44 +/- 6% relative to the pre-cardiopulmonary bypass state (p < 0.05). Over a range of cardiopulmonary bypass flows (2.3 to 1.7 L/min/m(2)), brain and kidney maintained their perfusion. Blood flow and O-2 delivery to both regions were reduced when the cardiopulmonary bypass now was reduced to 1.4 L/min/m(2). However, perfusion and O-2 delivery to other visceral organs (pancreas, small bowel) and skeletal muscle showed pump flow dependency over the range of flows tested. Conclusions. This study characterizes the organ-specific hierarchy of blood flow and O-2 distribution during cardiopulmonary bypass. These dynamics are relevant to clinical decisions for perfusion management.