In the interpretation of the phenomena of rubber-like elasticity the study of the changes in internal energy and entropy which accompany the extension are of particular interest. These quantities may be derived from the temperature dependence of the stress, or from the heat evolution on stretching. For natural rubber three regions may be distinguished, corresponding to (a) small strains (0 to ca. 10%), (b) intermediate strains (ca. 10 to 300%) and (c) large strains (> 300%). The statistical theory, in its simple form, requires a reduction of entropy on extension, with no change in internal energy. This requirement is satisfied only in the middle region (b). At higher and also at lower extensions the primary effect is obscured by secondary effects (volume changes and crystallization) which have no direct bearing on the mechanism of the deformation process. Further complications in the interpretation of thermoelastic data arise when the material being studied is able to absorb liquid from the surrounding medium, as frequently happens with biological systems. In such cases there are two equilibrium conditions to be satisfied: the mechanical equilibrium with respect to the applied forces, and the osmotic equilibrium with respect to the surrounding liquid. In general, the equilibrium liquid content will be a function not only of the temperature, but also of the applied stress. The dependence on stress has been verified for rubber swollen in various liquids, and for hair in contact with atmospheric water vapour. Finally, it may be noted that the thermodynamic analysis presupposes reversible behaviour. This condition is never completely satisfied; even with rubber, which is more perfectly elastic than most biological structures, the experimental difficulties arising from stress relaxation and other non-equilibrium effects have been extremely difficult to overcome.