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1. Heat and work production have been measured in pairs of frog sartorius muscles undergoing two periods of rapid isovelocity shortening at 0 degrees C. The first (conditioning) shortening occurred in the sarcomere length range 2.70--2.25 micrometer (as measured in resting muscles) and the second (test) shortening in the range 2.25--2.10 micrometer. The shortening heat associated with the test shortening was obtained as the difference in heat production between pairs of tetani which were identical except for the presence of the test shortening. 2. The shortening heat associated with the test shortening was reduced when it was preceded by the conditioning shortening; with no interval between shortenings its value was 52 +/- 3% (mean +/- S.E. of mean, n = 6) of that without the conditioning shortening. As the interval between shortenings was increased the shortening heat became larger; its recovery was more than half complete with an interval of 0.3 sec. 3. The work produced in the test shortening was also reduced in tetani which contained the conditioning shortening; its dependence on the interval between shortenings was similar to that of the shortening heat.