The deformation of a plastic material is normally accompanied by a change in temperature, which depends on the character of the process taking place. Low stresses cause small reversible strains with minimal time factors. In this regime there is general compliance with the classical laws of elasticity and the Joule-Thomson equation. Under tension the temperature drops, and in compression it rises. Both these changes can be measured quite accurately, from which the ratio of the coefficient of linear expansion to the specific heat may be derived. When the applied stresses are increased, large time-dependent deformations take place which are accompanied by an increase in temperature. A method of estimating this temperature change is reviewed, in which the temperature distribution in a tensile test piece under necking conditions is measured using infrared thermovision. The results are shown to follow a semi-empirical double-reciprocal relation between the rise in temperature and the rate of extension, from which an estimate can be made of the proportion of the applied work which is converted into heat. It is concluded that, under the conditions used, most of the applied mechanical energy appears as heat and there is an indication that the measured heat increase may even exceed the predicted level. Possible reasons for this effect are discussed.