We investigated the effects of flow rate and dopamine on systemic oxygen delivery (DO2) oxygen consumption (VO2) and gastric mucosal microcirculatory blood flow (gMCF), measured by laser Doppler flowmetry in 12 patients undergoing mild hypothermic (34 degrees C) cardiopulmonary bypass (CPB). The first intervention comprised increasing CPB flow rates from 2.4 to 3.0 litre min-1 m-2, and the second intervention administering dopamine 6 micrograms kg-1 min-1. Measurements were made before and 10 min after the start of one of the two interventions. The heart remained in cardioplegic arrest throughout the study. There were no significant differences in variables between the two baseline measurements preceding the interventions. The increase in CPB flow rate increased DO2 and gMCF without affecting VO2. At constant flow rate, dopamine also increased gMCF with no change in VO2, DO2 or mean arterial pressure. Our data suggested that dopamine had no flow-independent effect on VO2 and that it increased gMCF during constant flow hypothermic CPB.