We previously reported the serum nortriptyline levels of a series of breastfeeding mother-infant pairs. Nortriptyline was below the level of detectability in the infants' sera; however, two young infants aged 10 weeks or less had low concentrations of 10-hydroxy-nortriptyline. Because very young breastfeeding infants are likely to be at increased risk for toxicity, we have focused our study on breastfeeding newborns. We present additional data from six mothers and their infants (4 weeks of age), as well as data from one prematurely born baby. We were able to quantify nortriptyline in one infant and another had higher levels of hydroxymetabolites than previously reported, although still very low. No adverse clinical effects were observed in the infants.