We studied the influence of inotropic factors on the shape of the relation between tension and sarcomere length.Tension measurements were performed on thin trabeculae dissected from the right ventricle of the rat heart. Sarcomere length was measured by laser diffraction techniques and controlled by a servomotor system. The relations between tension and sarcomere length were derived from contractions at various extracellular calcium concentrations [Ca2+]o. The time course of tension development was dependent on both sarcomere length and [Ca2+]o. At all [Ca2+]o, the tension attained during contraction was zero at sarcomere lengths of 1.55-1.60 pm and maximal at a sarcomere length of 2.35 /im. Neither a summit nor a descending limb was found in the sarcomere length-tension relation. At [Ca2+]o= 0.5 mM, tension increased linearly with sarcomere length, whereas at [Ca2+]o= 2.5 mM, it approached maximal tension exponentially with sarcomere length. The relations between tension and sarcomere length derived from isometric contractions of the muscle and of sarcomeres were identical, and this suggests that shortening of sarcomeres does not contribute significantly to the effect of [Ca2+]o. The relations between tension and sarcomere length obtained at [Ca2+]o= 0.5 mM from contractions 30 seconds after a potentiating burst of stimuli (4 seconds at 4 Hz) were identical to the relation between tension and sarcomere length at [Ca2+]o= 2.5mm. Our results are consistent with the hypothesis that cardiac muscle length affects contractile performance by its influence on excitation contraction coupling.CircRes 46: 703-714, 1980