An isolated amphibian cardiac muscle preparation, toad ventricular strip, was used to examine the energetics of shortening. Simultaneous measurements of force and length changes and the associated heat production were made. Both the isometric heat/stress and the enthalpy (heat+work)/load relationships were similar to those previously reported in mammalian cardiac muscle. The activation metabolism was higher in this preparation and, like its mammalian counterpart, was length dependent. The heat production measured in an isometric contraction was approximately 50% higher than that observed at the same stress level in rodent mammalian cardiac muscle. This did not affect the maximum isotonic mechanical efficiency (work divided by enthalpy) of the preparation which, at an afterload of 20% of the maximum stress was 18.1 +/- 1.7% (n = 8). There was no evidence for a shortening heat component in this preparation during isotonic contractions. It appears therefore that the energetics of shortening amphibian cardiac muscle closely resemble the energetics of mammalian cardiac tissue.