Factors controlling the transfer of potentially toxic chemicals in the breast milk of nursing mothers include both chemical characteristics, such as lipophilicity, and physiologic changes during lactation. Physiologically based pharmacokinetic (PBPK) models can aid in the prediction of infant exposure via breast milk. Benefits of these quantitative models include the ability to account for changing maternal physiology and transfer kinetics, as well as the chemical-specific characteristics, in order to produce more accurate estimates of neonatal risk. A recently developed PBPK model for perchlorate and iodide kinetics in the lactating and neonatal rat demonstrates the utility of PBPK modeling in predicting maternal and neonatal distribution of these two compounds. This model incorporates time-dependent changes in physiologic characteristics and includes interactions between iodide and perchlorate that alter the distribution and kinetics of iodide.