In order to evaluate the effect of acute hypoxia on left ventricular (LV) contractility and diastolic function, hemodynamics and LV wall motion were investigated in anesthetized open-chest paced dogs using M-mode or pulsed Doppler echocardiography. Animals were ventilated with 10% oxygen (Hypo 1) and 6.3% oxygen (Hypo 2). LV contractility and diastolic functions were enhanced under "Hypo 1" and at an early phase of "Hypo 2". However, LV functions, both systolic and diastolic, were simultaneously reduced in the presence of hypercapnic acidosrs by "Hypo 2". Peak velocities of diastolic rapid filling flow (R) and atrial contraction flow (A) were increased under "Hypo 1", but showed a biphasic change (an increase and a subsequent decrease) under "Hypo 2". The ratio of A/R, known as an index of LV diastolic function, was not altered under hypoxia alone or even under hypercapnic acidosis. Even when hypoxia seems to enhance LV contractility, LV function has already begun to be de-pressed with a reduction of pH. This seems, however, to be compensated for by LV dilatation and increase in preload, or preservation of left atrial performance.