The development of energy metabolism of mammalian tissues was assessed in the tammar wallaby Macropus eugenii by the measurement of mitochondrial parameters in the liver, heart, kidney, and brain. Tissues taken from wallabies (n = 27) ranging from 10-day-old pouch young (weighing approximately 4 g) to adults (averaging 6.2 kg) were weighed and fixed, and mitochondrial volume and mitochondrial membrane surface area (MMSA) were determined by quantitative electron microscopy techniques. Developmental changes in these parameters were analyzed chronologically and allometrically. Relative growth rates of all four tissues decreased during development. Liver and heart showed constant allometric growth throughout development, whereas kidney and brain showed biphasic allometric growth. Tissue metabolic intensity assessed by MMSA (m2/cm3 tissue) was constant in liver, showed a threefold increase in brain during pouch life, showed a fourfold increase in the heart between 100 and 200 days of age, and showed a twofold increase in the kidney at the end of pouch life. In all tissues, adult levels of tissue metabolic capacity were present at pouch exit. In all four tissues, total MMSAs were at "reptilian" levels at birth and gradually increased to "mammalian" levels. Each tissue exhibited a different developmental timetable. When the total MMSAs for all four tissues were summed there was a similar pattern of allometric development between summed MMSA and whole animal metabolic rate.