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Department of Physiology, University of New England, Armidale, New South Wales, Australia.

1. Surfactant and fluid expressed from rabbit lungs has been used to form films on a simple Maxwell frame having a ratio of area-to-thermal capacity comparable to that existing in vivo. 2. When the area was increased by 2:1, the temperature fell by 5.6 degrees C and returned to 37 degrees C upon contraction as recorded by an infra-red thermometer with no thermal capacity. 3. The experiment was repeated upon thin lung sections when the temperature fall was 2.4 degrees C and was again reversed upon contraction. 4. When surfactant was removed from the surface of those sections, the temperature changed in the opposite direction, indicating that surfactant was responsible for the changes described above. 5. This surface energy phenomenon is discussed in relation to the common assumption that the lung operates under isothermal conditions when it may explain some of the discrepancies between studies of lung mechanics over different time scales. 6. Since the results are compatible with the inversion of hysteresis loops for surface tension versus the area of surfactant monolayers cycled to steady state in previous studies, it is speculated that heat, e.g. waste metabolic heat, might be the energy source for this surface 'engine' if, indeed, it is contributing to the work available for breathing.