The intracardiac hemodynamic responses to short periods of 70 degrees head-down and head-up tilts were studied in 12 normal male subjects, ages 19-24 years. Echo-Doppler techniques were used to measure the transmitral and transaortic flow velocities as well as cardiac index, and to evaluate the peripheral impedance. Head-down tilt (HDT) rapidly induced an increase (9.7%, p < 0.05) in the early passive filling of the left ventricle (ME peak of the transmitral flow velocity curve) and in transaortic flow velocity (8%, p < 0.05), as well as in cardiac output (6%, p < 0.05). In spite of a peripheral vasodilation, the blood pressure increased (7%, p < 0.05 for the systolic; 15%, p < 0.01 for the diastolic) and remained at a high level for the 5 min of the experiment. Head-up tilt (HUT) induced inverse responses; i.e., a large initial decrease in the transmitral (-15%, p < 0.05) and transaortic (-16%, p < 0.001) flows. The shape of the arterial peripheral flow indicated an increased vascular impedance. After a short drop, the blood pressure rapidly recovered a level statistically close to that of the pretest. In both cases, tachycardia occurred. We conclude that, in man, the cardiac responses to the changes in posture appear to be related more to the passive changes in ventricular filling due to the blood shift than to the nervous regulation by the arterial baroreflexes, whereas these reflexes mainly act in the control of the vascular impedance.