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The aim was to evaluate the relative contribution of atrial muscle contraction and atrial pressure to the phasic patterns of left atrial arterial and venous flows. Methods: Using a laser Doppler velocimeter, blood velocities were measured in the atrial small arteries and veins (outer diameter: 150-500 μm) in anaesthetised open chest dogs (n=21). The velocity sensor was fixed on the vessel surface with a drop of cyanoacrylate glue when good quality Doppler signals were consistently observed. Left atrial pressure and the contractility of the left atrium were changed by premature ventricular contraction and by intracoronary injection of isoprenaline (0.5 μg), respectively. Results: Premature ventricular contraction increased left atrial pressure significantly during arterial velocity measurements from 8.1(SD 2.7) to 16.4(1.3) mm Hg and during venous measurements from 8.2(1.2) to 14.3(3.7) mm Hg. However, premature ventricular contraction did not change the blood velocity patterns, the maximum deceleration rate of the systolic velocity wave in arteries, or the maximum acceleration rate of the systolic velocity wave in veins. Although isoprenaline did not change the left atrial pressure, it decreased minimum arterial blood velocity during atrial systole, from 3.3(3.4) to -2.5(3.2) cm·s−1, and increased maximum venous blood velocity from 15.9(5.5) to 19.2(7.4) cm·s−1. Isoprenaline also increased both the maximum arterial systolic velocity deceleration rate, from 90(45) to 234(143) cm·s−2, and the maximum venous systolic velocity acceleration rate from 356(230) to 763(366) cm·s−2. Conclusions: (1) Left atrial pressure is not a major determinant of the blood flow patterns of the atrial arteries and veins, and therefore it may not closely reflect pressure around mural vessels. (2) Atrial contractility affects the blood flow patterns of the atrial arteries and veins. Cardiovascular Research 1992;26:1219-1225