The sinus node is said to be occasionally visible in the human heart, while some have denied this visibility. This histological study of the normal sinus node was made on 11 adults and 10 infants. The node was always located subepicardially along the terminal groove at the junction of the superior caval vein and the right atrium. When no fatty infiltration (all infants and 2 adults) or minimal fatty tissue (2 adults) was found over the epicardial side of the node, it was not visible grossly. When fatty infiltration was obvious, then the node could be identified grossly as a yellowish spindle-shaped structure along the terminal groove. This arrangement was observed in 7 adults. The majority of nodes were in lateral position, but the so-called horseshoe arrangement was seen in 2 infants and 2 adults. The artery to the sinus artery was located centrally or eccentrically in the majority of cases, but the artery ramified through the nodal substance in 1 infant and 4 adults. We conclude that the node is not visible grossly in either the infant hearts or in those adult hearts without fatty infiltration. The whole cephalic part of the terminal groove should be carefully avoided to be sure of preventing the injury to the node during cardiac surgery.