The relations of the light chains of myosins of the atria, ventricles, and atrioventricular conducting tissue (specialized myocardial tissue) and the distribution of the light chains of myosin in different regions of the atrioventricular conducting tissue in bovine heart were examined. Two-dimensional gel electrophoresis showed that the atrial and ventricular myosins each had two light chains (LC1 and LC2). Ventricular LC1 differed from atrial LC1, but ventricular LC2 corresponded to atrial LC2. The specialized myocardial tissue myosin had three light chains (named here SL1, SL2, and SL3). SL1 comigrated with ventricular LC1, SL2 with atrial LC1, and SL3 with ventricular LC2 and atrial LC2. The compositions of the three light chains of myosins in various regions of the atrioventricular conducting tissue were determined by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. The percentage proportion of SL1 decreased in the order--atrioventricular node (AVN), right and left bundle branches (RLBB), His bundle (HIS), false tendon (FT) myosin; while the percentage proportion of SL2 decreased in the order--FT and HIS, RLBB, AVN myosin. The percentages of SL3 in these four regions were similar. The Ca2+-activated ATPase activity of myosin was highest in the AVN and lowest in the FT. The activities in the HIS and RLBB were intermediate between those in the AVN and FT. Thus, the composition of the light chains and the Ca2+-activated ATPase activity were different in various regions of the atrioventricular conducting tissue.