The purpose of the present study is to assess the influence of upper thoracic epidural anesthesia (TEA) on hemodynamics, the systemic oxygen supply-demand relationship, and several indications of stress in the presence of preexisting hypovolemia. Sixteen adult mongrel dogs were anesthetized with halothane and nitrous oxide, and blood was withdrawn at the rate of 3 ml.kg-1.min-1 for 10 min. The dogs were randomly assigned to two groups; one receiving TEA (TEA group, n = 8), and the other without (controls, n = 8). Mean arterial blood pressure, heart rate, arterial concentrations of epinephrine and norepinephrine remained significantly lower (P less than 0.05) in the TEA group than in the controls during the 100-min observation period following the start of TEA. Cardiac index, oxygen supply-demand ratio (O2 delivery/O2 consumption) and arterial lactate concentration showed no significant intergroup differences at any point during observation. By contrast, there was a significant difference in arterial pH between the TEA group (7.22 +/- 0.05, mean +/- SD) and the controls (7.15 +/- 0.08) after 100 min of TEA. These results indicate that in hypovolemic dogs anesthetized with halothane and nitrous oxide, TEA affects the acid-base balance favorably without significantly influencing the systemic oxygen supply-demand relationship.