This study compared the contractile performance of a canine right atrial trabecula with that of a macroscopically indistinguishable trabecula isolated from the right ventricular apex. The heart was removed from the nine mongrel puppies weighing 6-8 kg and placed in Krebs Ringer's bicarbonate solution. The bathing solution contained only 1.25 mmoles of Ca2+ and was bubbled with a 95% O2-5% CO2 gas mixture. Each atrial trabecula was specially selected from the right atrial appendage. Histologically, these trabeculae showed a remarkable longitudinal orientation of the fibers. At Lmax (the length of the muscle at which developed tension was maximum) under identical conditions of temperature, rate of stimulation, ionic milieu, pH, and O2 and CO2 supply, right atrial trabeculae achieved the same developed and total tensions but in a much shorter time than did ventricular trabeculae. In both muscle groups the maximum developed tension averaged about 2.5 g/mm2. Sinc Lo (expressed as a fraction of Lmax) was less in atrial muscle than it was in ventricular muscle, we concluded that atrial muscle can be stretched considerably more than can ventricular muscle before optimum length is reached. At any given initial muscle length, the maximum of tension rise for atrial trabeculae amounted to at least twice that for ventricular trabeculae. At any given load up to 1.5 g/mm2, the maximum velocity of shortening of an atrial trabecula was about three to four times that of a ventricular trabecula. These results collectively indicate that the contractile performance of the right atrial muscle is in many respects superior to that of the right ventricle, at least under the conditions of these experiments.