The left and right atria of the mouse were compared to each other and to the mouse left ventricle using stereologic techniques. The volume fraction (Vv) and surface area per unit cell volume (Sv) of the interior junctional sarcoplasmic reticulum (IJSR), total JSR and extended JSR were greater in the left atrium than in right. The Vv and Sv of the free SR, transverse tubules, and mitochondria were similar in the two atria. It is suggested that the differences in junctional sarcoplasmic reticulum between the atria can be accounted for by a difference in distribution of two types of cells whose anatomy is analogous to working and conducting fibers in the ventricle. The Sv and Vv of the transverse tubules, mitochondria, and all the components of the sarcoplasmic reticulum except for the free SR were greater in the left ventricle than in either atrium. The greater calcium content and sensitivity to extracellular calcium of the atria may explain the greater volume of free SR in the atria as compared to the left ventricle. The Sv of the plasmalemma of the atria and of the Sv of the plasmalemma of the transverse tubules of the left ventricles supports the suggestion of others that there is a constant ratio of surface area to cell volume in cardiac cells.