1. Dynamic changes in left atrial (LA) function during treadmill exercise were studied in ten conscious dogs instrumented to measure left ventricular (LV) pressure and diameter, LA pressure and diameter, and pulmonary venous blood flow (PVF, transit time flowmeter). 2. Systolic PVF volume (reservoir volume; a measure of LA reservoir function) increased from 38 +/- 4% of total PVF volume at baseline to 52 +/- 8% of total PVF volume during exercise, and diastolic PVF volume (conduit volume; a measure of LA conduit function) decreased from 62 +/- 5% at baseline to 48 +/- 8% during exercise (P < 0.005). 3. The increases in reservoir volume and the decrease in conduit volume were due not only to a greater decrease in diastolic interval than systolic interval but were also caused by a significantly greater increase (P < 0.05) in the mean systolic filling rate (93%) than in the mean diastolic filling rate (51%). 4. During exercise the pattern of LV filling derived from changes in LV diameter showed that a greater percentage of LV filling occurred during the second half of diastole at the time of atrial contraction (P < 0.05), suggesting that LA booster function was enhanced. 5. Changes in LA dimension revealed that during exercise more blood volume was reserved in the LA during systole and that this change was associated with an increase in the LA dimension at the beginning of LA contraction (r = 0.61, P < 0.05). 6. We conclude that LA reservoir and booster functions were augmented during exercise, whereas conduit function was not. Increased reservoir function may play an important role in accelerating LV filling by helping to maintain an enhanced atrioventricular pressure gradient during diastole and also by increasing LA booster function through an increase in LA preload.