A deformation microcalorimeter was built, which allows simultaneous measurements of the very small heat and work contributions of living single muscle fibres on quick-stretch with a temporal resolution of 1 ms. The thermodynamic behaviour of small muscle-fibre bundles of Iliofibularis and Tibialis anterior muscles of Xenopus laevis frogs have been investigated in the relaxed and active states. In particular, twitch, tetanus and quick-stretch experiments have been performed. An exothermic heat production is strongly correlated to the changes in force and almost consumed again when the force decreases after stimulation. In the case of active quick-stretch, then are two heat events clearly distinguishable in time, the first exothermic and the second endothermic. The amount of heat is equal in both the events, but with opposite sign, and approximate to 20 times larger than the work done during stretching. One event is suspected to be an elastic heat, the other may be due to an activated process (i.e. a reversible chemical or folding/unfolding reaction of proteins).