Summary.A simple, sensitive, and highly specific method has been developed for determination of valacyclovir (VL) in human plasma. The analytical procedure involves a solid-phase extraction method using Valacyclovir-D8 (VLD8) as an internal standard. Chromatographic separation was carried out on a reversed phase Zorbax, SB C18, 4.6 × 75 mm, 3.5 μm column. Valacyclovir and Valacyclovir-D8 were detected with proton adducts at m/z 325.2 → 152.0 and 333.3 → 152.0 in multiple reaction monitoring (MRM) positive mode. The method was linear over the concentration range of 0.5-700.0 ng mL −1 . The limit of detection (LOD) and limit of quantification (LOQ) for valacyclovir were 0.2 pg mL −1 and 0.5 ng mL −1 , respectively. The method was shown to be precise with the average within-run and between-run variations of 0.7 to 3.5% and 3.1 to 4.7%, respectively. The average within-run and between-run accuracies of the method throughout its linear range were 96.7 to 97.9 and 94.7 to 97.3%, respectively. The mean recoveries of valacyclovir and Valacyclovir-D8 from human plasma by the developed method were 99.17 ± 10.78% and 110.84 ± 8.74%, respectively. The method was successfully applied in bioequivalence study with 20 healthy male volunteers under fasting condition.