The artery of the sino-atrial node was studied in 100 normal human hearts after injection of each coronary artery with coloured gelatine containing a radiopaque substance. The hearts belonged to 69 males and 31 females, being 64 Caucasians and 36 non-Caucasians (Negroes and Mulattoes) whose age ranged from 7 to 80 years. Since the individuals had committed suicide or were victims of accidents, their hearts, after pathologists' evaluation, were considered normal. The sinoatrial node of the normal human heart is supplied by the right coronary artery more frequently (58% +/- 4.9% of the cases) than by the left (42% +/- 4.9). The right anterior medial atrial artery, originating from the right coronary at the level of the medial third of the right anterior quadrant of the atrial dome, is most frequently (50% +/- 5) responsible for the blood supply of the sinoatrial node. Among the branches of the left coronary artery, the left anterior medial atrial artery, originating at the level of the medial third of the left. anterior quadrant of the atrial cupola, was the most frequent blood supplier (25% +/- 4.3) of the sinoatrial node. The origin of the artery of the sinoatrial node from the proximal portion or trunk of the left coronary artery was less frequent (12% +/- 3.2) than the origin from the circumflex artery (30% +/- 4.5). Neither sex nor race influenced the variations of the origin of the sino-atrial node.