The concentration of total carnitine in human breast milk remained at a constant mean level near 62.9 range: 56.0-69.8/nmoles/ml during the first 21 days postpartum. The carnitine level fell significantly to 35.2 +/- 1.26 nmoles/ml until the 40-50th day. The carnitine concentrations did not depend on the secreted volume of milk. In comparison, fresh and commercial pasteurized cow's milk contained 206.2 (range: 192-269) nmoles/ml and 160.0 (range: 158-200) nmoles/ml carnitine, respectively. In the sera of nursing mothers, the concentration of total carnitine was lower on the first day after delivery (27.2 +/- 1.19 nmoles/ml) but returned to normal by the 21st day postpartum (38.8 +/- 2.97 nmoles/ml).