Objective: The purpose of this study was to determine the valaciclovir and acyclovir pharmacokinetic profiles in serum and breast milk after valaciclovir administration to women after delivery. Study Design: Valaciclovir (500 mg twice daily for 7 days) was given to 5 women after delivery who were breast-feeding healthy term infants. Matched serum and breast milk samples were obtained after the initial dose, on day 5 and 24 hours after the drug was discontinued. Infant urine was obtained on day 5. Results: Valaciclovir was rapidly converted to acyclovir. The peak serum acyclovir concentration occurred 3 hours before the peak breast milk concentration (2.7 μg/mL at 1 hour vs 4.2 μg/mL at 4 hours). The serum acyclovir elimination half-life was 2.3 hours. The ratio of breast milk to serum acyclovir concentration was highest 4 hours after the initial dose at 3.4 and reached steady state ratio at 1.85. The median infant urine acyclovir concentration at steady state was 0.74 μg/mL. Conclusion: Valaciclovir is rapidly converted to acyclovir and concentrates in breast milk. However, the amount of acyclovir in breast milk after valaciclovir administration is considerably less (2%) than that used in therapeutic dosing of neonates. (Am J Obstet Gynecol 2002;186:100-2.)