Microcalorimetric investigations of living cellular systems have been conducted on small animals, cells and tissues from animals and plants and on microbial systems, but it is only recently that more systematic work has begun on plant materials. Isothermal experiments together with temperature scanning experiments have been performed. In most cases simple static ampoules have been used as calorimetric vessels and practically all calorimetric investigations reported on plant systems have been conducted under dark conditions. It is felt that some microcalorimetric techniques currently used in studies of microbial and animal systems, and in other areas, could be usefully applied more generally in the plant field : stirred injection (titration) vessels, gas and liquid perfusion vessels and different photocalorimetric methods.