The restriction imposed by mitral stenosis on the cardiac output response to exercise provides a unique opportunity in man to evaluate determinants of coronary flow and myocardial oxygen consumption, since one or several variables may be limited. Coronary and systemic hemodynamic patterns were therefore evaluated in 12 patients with mitral stenosis and compared with normal subjects.While cardiac output and left ventricular work were lower than normal at rest, coronary flow and myocardial oxygen consumption did not differ from normal. During exercise, despite subnormal cardiac output and left ventricular work responses, coronary blood flow and myocardial oxygen consumption increased normally in the group of patients in whom arterial pressure rose. However, coronary flow fell in four patients who experienced exertional hypotension, while myocardial oxygen consumption was reduced or unchanged. In both groups, myocardial oxygen extraction was above normal at rest and rose further during exercise. Changes in myocardial oxygen consumption followed alterations in the pressure-time index, but the excessive heart rate response to exercise resulted in a greater oxygen expenditure per unit of tension developed per beat than that found in normal subjects.It is concluded that the primary determinant of coronary blood flow in mitral stenosis is the arterial perfusion pressure while myocardial oxygen consumption is primarily dependent on the pressure-time index. In addition, heart rates inappropriately high for a given state result in energy expenditures above that required for maintenance of pressure and cardiac output.