Concerns for potential vulnerability to manganese (Mn) neurotoxicity during fetal and neonatal development have been raised due to increased needs for Mn for normal growth, different sources of exposure to Mn, and pharmacokinetic differences between the young and adults. A physiologically based pharmacokinetic (PBPK) model for Mn during human gestation and lactation was developed to predict Mn in fetal and neonatal brain using a parallelogram approach based upon extrapolation across life stages in rats and cross-species extrapolation to humans. Based on the rodent modeling, key physiological processes controlling Mn kinetics during gestation and lactation were incorporated, including alterations in Mn uptake, excretion, tissue-specific distributions, and placental and lactational transfer of Mn. Parameters for Mn kinetics were estimated based on human Mn data for milk, placenta, and fetal/neonatal tissues, along with allometric scaling from the human adult model. The model was evaluated by comparison with published Mn levels in cord blood, milk, and infant blood. Maternal Mn homeostasis during pregnancy and lactation, placenta and milk Mn, and fetal/neonatal tissue Mn were simulated for normal dietary intake and with inhalation exposure to environmental Mn. Model predictions indicate similar or lower internal exposures to Mn in the brains of fetus/neonate compared with the adult at or above typical environmental air Mn concentrations. This PBPK approach can assess expected Mn tissue concentration during early life and compares contributions of different Mn sources, such as breast or cow milk, formula, food, drinking water, and inhalation, with tissue concentration.