The physical properties of a vacuum-deposited thermopile of antimony and bismuth with 69 hot junctions on 18 mm of a thin capton foil are presented. Using this, temperature changes down to a threshold of 0.15 mK with a rise time of 1.5 ms may be measured. The heat transfer properties have been both measured and simulated with the aid of the finite element method. This thermopile is suitable as a quasi-adiabatic calorimeter for fast heat measurements of small objects. In particular, it is possible to measure heats of single muscle fibres in quick-stretch experiments down to 1 mu J with a time resolution of 0.2 ms.