1. The change in the relative contribution of the early passive and later active phases of transmitral flow to left ventricular filling was studied using Doppler echocardiography in ten normal male subjects during mild exercise. 2. The peak velocity of passive flow increased during exercise by a mean of 16% whereas peak velocity of active flow increased by a mean of 89%. Hence the ratio of the peak velocities decreased in a linear fashion with a correlation coefficient of r = -0.95. 3. The ratio of the Doppler-derived velocity-time integrals (equivalent to the ratio of flow) of the two phases of transmitral flow also showed a significant negative linear correlation of r = -0.97. 4. Active atrial transport (atrial systole) progressively increases its contribution to overall transmitral blood flow with increasing heart rate during mild exercise. This effect is mainly mediated by an increase in flow velocity which is related to increased atrial contractility.