Changes in the sinoatrial node represent the major mechanism of sudden death in humans, and because of the sparse knowledge about the effects of aging on this structure, light microscopic and quantitative studies of the sinoatrial node were undertaken. Twenty-one hearts were studied, seven rat hearts from each of the following age groups: three months of age, twelve months of age and eighteen months of age. In the stereo logic study, the following parameters were studied: Vv_[nc] and Vv_{[interstitium]} % (the volume densities of the nodal cell and interstitium, determined by the point-couting method), and Nv_[nc](1/mm³) (the numerical density of the nodal cell, determined by the disector method). The mean volume of the nodal cell (V_[nc]) (µm³) was also determined. The comparisons showed that in the oldest animals, the volume density of the nodal cells decreased, while the volume density of the interstitium increased. Although numerical density of the nodal cell per volume of sinus node decreased, the nodal cells displayed increased mean volume with age. In conclusion, the aging process implies changes in the cell and fiber content of the sinoatrial node.