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1. The effect of repeated stimulation and of FDNB on the heat of shortening has been studied in sartorius muscles of Rana temporaria at 0° C.

2. The muscles were stimulated at 15 min intervals during 1·2 sec. During the plateau of tetanic activity, they were allowed to shorten 4·3 mm at constant velocity. After eight such contractions, they were poisoned with 0·4 mM-FDNB and the series continued.

3. From tetanus to tetanus, the tension during shortening decreased faster than the isometric tension; FDNB enhanced this effect. Despite a progressive decrease in the initial heat production, the heat of shortening did not change significantly in the normal muscle; immediately after poisoning it became larger, but from then on, it declined rapidly from tetanus to tetanus, faster than the over-all heat production.

4. The first tetanus after FDNB is thus especially favourable when looking for a chemical counterpart of heat of shortening. On the other hand, the postulated development of a friction in the intoxicated muscle cannot explain the changes in mechanical and thermal properties of the muscle. This point is discussed.