The effects of ageing and starvation on the rat myocardium were studied by morphometric methods. Since cardiac muscle is a tissue with a high level of anisotropy, methods based on the concept of vertical planes were used to describe quantitative alterations in the rat myocyte both at the cellular and ultrastructural level. During starvation rapid and important changes were noted, particularly in the transverse dimension of cells and organelles. The most striking change, however, was the immediate dilatation of the myocyte T-system, reflecting an adaptive interaction between the intra- and extracellular environment. At the same time exocytosis of intracellular components into the extracellular space of the T-system was observed. The ratio of mitochondria to myofibrils decreased progressively during starvation. Such a decrease, in general, may reach a point when cellular energy supply becomes compromised. A comparison between different regions of the heart showed no differences and it can be concluded that the morphological changes during starvation are the same, and equally distributed, in both ventricles. The changes described in the aged rat heart point in the direction of a hypertrophy of the aged myocyte. This leads to a lower ratio between surface and volume which finds its representation at the subcellular level in a more spherical shape of nuclei and mitochondria. Unlike what is seen in malnutrition, the mitochondrial/myofibril ratio is higher in the older rat. From the morphological point of view, the atrophy of malnutrition and the hypertrophy of ageing are opposed, but in both there is a change in the relationship of the myocyte to its environment which directly influences the substrate exchange capacity. This tends to protect the myocyte in starvation but jeopardizes the older cell.