The exact location of the sinus node was determined in 25 infant hearts using reconstructive techniques. The sinus node was found to lie on the lateral aspect of the superior cavo-atrial junction in the great majority (22 hearts). In only 3 hearts was it found to straddle the summit of the crest of the right atrial appendage where it is popularly believed to be located. In the majority of instances a single artery supplied the node (in 11 hearts the artery approached posteriorly, and in 10 the artery approached anteriorly), though there was considerable variation in the exact anatomy of the arterial supply. Connections between the sinus node and atrial myocardium were diffuse and abundant but in no instances were tracts of specialised cells observed leaving the node and traversing any distance through the atrial wall. These findings are relevant to the sinus node dysfunction that may follow cardiac surgical procedures.