Marked changes in intermediary metabolism occur during development of the heart. In the fetus, the heart utilises lactate and glucose as its main energy substrates, while in the adult, fatty acids are the main energy substrate. The transition from carbohydrate to fatty acid metabolism is a complex process which involves maturation of mitochondrial processes and dramatic changes in circulating levels of fatty acids and lactate. In addition, developmental changes in the use of energy substrates also involve changes in the regulation of the enzymes involved in both carbohydrate and fatty acid utilisation. This paper reviews these changes in intermediary metabolism which occur during myocardial development. The metabolic differences that exist between immature and adult hearts may explain the observed differences in the ability of immature hearts to withstand hypoxaemia or ischaemia.