Heat capacity measurements have been performed on birch plants (Betula pendula Roth.) by use of a drop-Cp-microcalorimeter, Influence of hydration level on heat capacity was investigated. It was found that heat capacity decreased in a linear fashion with decreasing water content until a point of discontinuity was reached at a water fraction of 0.2 g g(-1) (grams of water per gram fresh biomass). A linear regression of the region above 0.2 g g(-1) gave a slope of 2.49 J K-1 g(-1) and an intercept of 1.72 J K-1 g(-1.) It was concluded that a rough division of the water in the biomass into free and perturbed water is reflected in the heat capacity of the system and that the heat capacity in the region below 0.2 g g(-1) could be divided into three discrete subregions, These subregions are assumed to reflect the perturbed water state in the tissue at different levels of hydration. Further, the temperature influence on heat capacity was studied in the range between 14 and 37 degrees C. The water fraction in these experiments was 0.75+/-0.1 g g(-1). No significant difference in heat capacity values was observed. It was concluded that this result reflects the dominant influence of free water on heat capacity at this level of water content.