To assess carnitine levels during prolonged sole breast feeding we measured serum and breast milk carnitine concentrations in 37 lactating mothers and their healthy term infants from birth to the age of 1 yr. The number of solely breast-fed infants decreased to 31 at 2 months of age, to 28 at 6 months, and to seven at 9 months, because formula and/or solid food was added when there was not enough breast milk. In mothers the mean serum carnitine increased from 35 to 50 mumol/liter during the first 2 months after delivery and remained unchanged thereafter. Irrespective of the type of feeding, the mean serum carnitine in infants increased from 29 to 59 mumol/liter during the first 2 months, remained unchanged during 2-9 months, and decreased to the mean level of mothers thereafter. The mean carnitine concentration of breast milk was high (106 mumol/liter) immediately after delivery. During the first 2 months the mean carnitine concentration of milk decreased to the mean serum level of mothers and remained unchanged thereafter. The carnitine concentrations of serum and breast milk did not correlate, however. The mean daily carnitine intake of the breast-fed infants was 5.7 mumol/kg at 4 months of age, 4.7 mumol/kg at 6 months, and 6.0 mumol/kg at 9 months whereas the mean daily carnitine intake of the infants receiving formula was 28.9 mumol/kg at 1 month of age and 30.7 mumol/kg at 2 months. The serum concentration of carnitine in our infants did not correlate with carnitine intake. Our results indicate that serum carnitine concentrations are maintained during prolonged sole breast feeding.