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1. Pairs of frog sartorius muscles were stimulated for 2 sec at 0 degrees C, after 1 sec of isometric contraction, were released at a constant velocity. The total excess heat (shortening heat) and work associated with the release were determined by comparison with isometric control tetani. 2. Shortening heat and work production were non-linearly related to the distance shortened. There was proportionally more energy liberation for smaller releases. 3. The dependence of shortening heat on muscle length was investigated within the sarcomere length range 2.1--2.6 micrometer (as measured in resting muscle) and was found to be similar to that of isometric tension. 4. A simple model in which heat and work are produced in a two-state cycle can describe these and previous results concerning the energetics of rapidly shortening muscle.