The light chains of myosin from atrial and ventricular tissues from rat and rabbit were examined by one- and two-dimensional polyacrylamide gel electrophoresis. The myosin heavy chains were electrophoretically isolated, digested after denaturation in sodium dodecyl sulfate with papain and proteinase from S. aureus V8, and the resulting peptides resolved in one-dimensional gel electrophoresis. The peptide patterns of myosin heavy chains from atrial and ventricular tissues of adult rabbits were different, indicating differences in their primary structures. No such differences could be detected in a total of around 180 peptides produced by the two proteinases from the myosin heavy chains of adult rat atrial and ventricular tissues. With regard to light chains, the same migration pattern was observed for atrial and ventricular tissues from both rat and rabbit. The atrial light chains ALC1 and ALC2 migrated with molecular weights lying between those of the ventricular light chains VLC1 and VLC2. In two-dimensional electrophoresis, the corresponding light chains from rat and rabbit co-migrated. An additional light chain was observed in foetal ventricles, which exhibited identical electrophoretic properties to ALC1 from adult atrial tissues. In rat myofibrillar preparations from atrium and ventricle, an unidentified protein (x) occurred in the region of light chain-1 but with a more acidic isoelectric point, which seems to be related to the developmental stage of these tissues and which could not be detected in rabbit heart tissues or in any skeletal muscles.