BACKGROUND: Varying results of hemodynamic and oxygen parameters in response to hypoxia and/or hypercarbia have been reported. In this study, the effects of apnea on the hemodynamic parameters and oxygen availability were evaluated using ten healthy mongrel dogs. METHODS: After full oxygenation, apnea was induced by disconnecting animals from mechanical ventilation. Hemodynamic parameters, oxygen delivery, consumption and extraction ratio were measured at one minute intervals until the cardiac output was undetectable via the thermodilution method. RESULTS: Blood pressure (BP) increased continually following apnea. Cardiac output (CO) increased during the early of apnea (2 and 3 minute) but decreased thereafter. Systemic vascular resistance (SVR) decreased in the early phase of apnea but subsequently declined. The indices of preload increased steadily following apnea. Changes in heart rate (HR) were compared with changes in CO and oxygen delivery, and was found to increase during the early phase of apnea (2~3 minutes) and decrease thereafter. The oxygen extraction ratio did not change significantly and remained steady 6 minutes after apnea but increased and became irregular thereafter. CONCLUSIONS: These data suggest that early reflex responses such as increased BP were primarily due to increased CO, whereas the late increase in BP was due to the increase of SVR. We conclude that BP is not meaningful variables in evaluating critical hypoxic condition such as apnea, and bradycardia might be caused by decreased CO and severe hypoxemia.