The purpose of our study was to assess the influence of lactation stage-dependent differences in milk-to-serum (M/S) ratio for an actively transported drug (cefepime) on the calculation of 2 exposure indices, EI(Dose) and EI(Conc). Age-dependent differences in cefepime pharmacokinetics at postnatal days 4 and 10 as well as cefepime M/S values at lactation days 4 and 10 in rats were determined. Significantly higher elimination rate constant and systemic clearance and lower half-life were found in day 10 compared to day 4 pups with no differences in oral bioavailability. The EI(Conc) was quantitatively higher than EI(Dose) at both lactation stages. The approximately 7-fold decrease in cefepime M/S values at lactation day 10 resulted in approximately 7-fold reduction in the EI(Dose) and approximately 13-fold reduction in EI(Conc). Our study confirms the need to evaluate M/S at different lactation stages for actively transported drugs to avoid over- or underestimation of neonatal exposure risk.