We analyzed blood flow pattern in the interatrial communication in 24 patients with complete transposition of the great arteries (TGA). Eight had TGA with atrial shunt (group 1), nine had TGA with patent ductus arteriosus or ventricular septal defect (group 2), and seven had pulmonary arterial banding and Blalock-Taussig shunt (group 3). The flow pattern was determined at the site of atrial septal defect by Doppler echo beam directed as perpendicular to the septum as possible. The flow pattern was composed of a left-to-right (L-R) flow and right-to-left (R-L) flow. The turning point (T1) from the R-L to L-R flow occurred immediately after the initiation of the QRS on the electrocardiogram and was common in all groups. The other turning point (T2) from L-R to R-L occurred after the second heart sound (S2). The S2-T2 interval decreased on inspiration, indicating prolongation of the period of R-L flow. The minimum S2-T2 interval ranged from 20 to 70 (mean + SD 50 + 18) msec in group 1, from 70 to 130 (114 ±- 25) msec in group 2, and from 50 to 138 (75 29) msec in group 3. The maximum S2-T2 interval ranged from 48 to 110 (88 21) msec in group 1, from 140 to 235 (175 36) msec in group 2, and from 80 to 170 (111 30) msec in group 3. The minimum ratio of L-R flow duration to that in the whole cardiac cycle (TI-T2/RR) was 0.47 to 0.61 (mean ± SD 0.53 + 0.04) in group 1, 0.66 to 0.88 (0.74 + 0.10) in group 2, and 0.53 to 0.77 (0.65 0.08) in group 3. The maximum TI-T2/RR ratio ranged from 0.57 to 0.74 (0.67 + 0.06) in group 1, from 0.75 to 1.0 (0.87 + 0.09) in group 2, and from 0.68 to 0.91 (0.79 0.08) in group 3. We conclude that, in patients with TGA (1) an interatrial shunt occurs in a rather simple flow pattern, with L-R shunting mainly in systole and R-L shunting mainly in diastole, (2) the pattern is affected by respiration, and (3) the associated ventricular septal defect or patent ductus arteriosus causes the shunt, which directs flow toward the pulmonary artery and away from the systemic ventricle or artery.