Manganese balance studies have been performed on 16 infants, aged 3 days to 8 months, in the period following operation for the correction of congenital heart defects. Samples were analyzed by flameless atomic absorption spectroscopy or by solvent extraction with 8-hydroxyquinoline followed by flame atomic absorption spectroscopy. A higher manganese content was found in either whole blood (710 ± 320 nmole·litre-1) or purified plasma protein (1130 ± 770 nmole·litre-1) compared with fresh frozen plasma (215 ± 35 nmole·litre-1) used in intravenous drips. The manganese content of the milks used in oral feeding was 200–300 nmole·litre-1.