The heat produced by frog muscle in a series of contractions with shortening.

1. Heat production has been measured in muscles undergoing rapid shortening 1 sec after the start of stimulation in a 2 sec tetanus. In a series of three such tetani where the periods of shortening were separated by 5 sec the shortening heats in the second and third tetani were 111 (+/- 4)% and 116 (+/- 6)% respectively of that in the first tetanus (mean +/- S.E. of mean, nine experiments). 2. When a similar series was carried out after an interval of 3 min the shortening heat in the first tetanus of the second series was 127 (+/- 3)% of that in the first tetanus of the first series (mean +/- S.E. of mean, three experiments). 3. The results are not in agreement with those of Dickinson & Woledge (1974). The discrepancy is explained by an artifact which, with certain arrangements of the apparatus, affects the measurements of shortening heat in a series of contractions. We have measured this artifact by two independent methods; the shortening heat values quoted above have been corrected accordingly. 4. The increase in the shortening heat in repeated contractions is not a specific consequence of previous shortening; an increase of the same magnitude is observed following isometric tetani. 5. During the time when the muscle is shortening, the total rate of heat production, which is initially high, falls to a steady value. This value is not significantly different in the three tetani of the series.