The effects of insulin on myocardial contractility (MC) and metabolism were studied in 18 newborn lambs. Cardiac work and rate were kept constant. Following insulin (80 units, intravenously), contractility increased and remained elevated for 1 hour. This was not prevented by beta blockade or glucose infusion. Myocardial extraction and uptake of glucose and nonesterified fatty acid (NEFA) increased, the arterial concentration of both decreased. But after glucose fell to less than 20 per cent of control, uptake of both glucose and NEFA declined as did MC. It is concluded that in the neonate insulin increases MC as well as substrate uptake.