The objective of these experiments was to better understand the factors responsible for the decline in tension at short sarcomere lengths in cardiac muscle. We measured the effects of variation of extracellular calcium concentration on the sarcomere length-tension relation in thin trabeculae and papillary muscles excised from right ventricles of rats. Sarcomere lengths were measured by optical diffraction. Experiments were carried out using two protocols. In the primary protocol, the muscle was allowed to contract isometrically. The sarcomeres in the central region, shortened and stretched the end regions adjacent to the clips. The sarcomere length was measured at the time of peak tension and plotted against the active tension. In a secondary protocol, the sarcomere length and tension were measured during contractions in which sarcomere length in the central region was held approximately constant. Both protocols were carried out at extracellular Ca2+concentrations ranging from 0.3 to 5.0 IDM. The height of the length-tension curves was progressively depressed as extracellular Ca2+was reduced from 2.5 to 0.3 min; the variation of the shape of the curve was modest. On the other hand, when Ca2+was increased to 5 DM, there was less upward shift of the sarcomere length-tension relation, indicating a tendency toward saturation. The results obtained using the sarcomere isometric protocol were similar to those obtained with the muscle isometric protocol. Extracellular Ca2+appears to act principally as a modulator of the height of the length-tension curve, though it has a modest effect on the shape as well. On the basis of these results, we deduce that several of the factors previously cited as possible explanations of the decline in tension at short sarcomere length are not likely candidates.Circ Res 47: 610-619, 1980