The new inhalation anesthetic, Forane, has been shown to produce only minimal depression of cardiac output during controlled ventilation, but a significant decrease in arterial pressure and profound respiratory depression. This study was undertaken to determine whether spontaneous ventilation, with the consequent elevations in CO2, would alter the cardiovascular responses to Forane, and whether Forane alters the cardiovascular response to imposed increases in CO2(CO2challenge). During controlled ventilation Forane maintains cardiac output, primarily through an increase in heart rate at the expense of stroke volume. Spontaneous ventilation alters the cardiovascular response to Forane, in that heart rate and cardiac output are further increased. These changes can be explained by the increase in Paco2secondary to spontaneous ventilation. Cardiovascular CO2response slopes reveal that Forane decreases the response of the cardiovascular system to increases in Pco2As in previous studies with Forane, the authors did not find any stimulation of the cardiovascular system with time.