Several parameters indicative of whole muscle aerobic and anaerobic metabolism were compared in heart, diaphragm and two locomotory muscles of sheep (Ovis aries) ranging from 90 days gestation to adult animals. Aerobic metabolism was assessed from myoglobin content, anaerobic metabolism from the pH buffering capacity and the balance between the two from the properties of the lactate dehydrogenase isozymes. We expected the diaphragm and heart, as the two vital pumps of the body, to have substantial aerobic capacity at birth. For the left ventricular myocardium this appears to be true, with the myoglobin level at birth averaging 69% of the adult value. However, the diaphragm had only 15% of the adult myoglobin level, a level similar to that in vastus lateralis and medial gastrocnemius muscles. In the first 18 days postnatally, myoglobin levels in the diaphragm did not change. Anaerobic capacity, as indicated by pH buffering capacity, increased in all four muscles during fetal life, reaching from 69-96% of adult values on day 1 after birth. A rapid increase in pH buffering capacity occurred in all four muscles in the first 6 days postnatally suggesting that anaerobic glycolysis is important to the newborn. We conclude that the work done by the fetal heart leads to the development of a high aerobic capacity by birth, but the activity of the fetal diaphragm does not have this effect.