Valproic acid (VPA) is a widely used anticonvulsant that is also approved for mood disorders, bipolar depression, and migraine. In vivo, valproate is metabolized oxidatively by cytochromes P450 and β-oxidation, as well as conjugatively via glucuronidation. The acyl glucuronide conjugate (valproate-glucuronide or VPAG) is the major urinary metabolite (30-50% of the dose). It has been hypothesized that glucuronidation of antiepileptic drugs is spared over age, despite a known decrease in liver mass. The formation rates of VPAG in a bank of elderly (65 years onward) human liver microsomes (HLMs) were measured by liquid chromatography/tandem mass spectrometry and compared with those in a younger (2-56 years) HLM bank. In vitro kinetic studies with recombinant UDP-glucuronosyltransferases (UGTs) were completed. A 5- to 8-fold variation for the formation of VPAG was observed within the mi-crosomal bank obtained from elderly and younger donors. VPAG formation ranged from 6.0 to 53.4 nmol/min/mg protein at 1 mM substrate concentration (n = 36). The average velocities at 0.25, 0.5, and 1 mM VPA were 7.0, 13.4, and 25.4 nmol/min/mg protein, respectively, in the elderly HLM bank. Rates of VPAG formation were not significantly different in the HLM bank obtained from younger subjects. Intrinsic clearances (VmaxKm) for several cloned, expressed UGTs were determined. UGT1A4, UGT1A8, and UGT1A10 also were found to catalyze the formation of VPAG in vitro. This is the first reported activity of these UGTs toward VPA glucuronidation. UGT2B7 had the highest intrinsic clearance, whereas UGT1A1 demonstrated no activity. In conclusion, our investigation revealed no differences in VPAG formation in younger versus elderly HMLs and revealed three other UGTs that form VPAG in vitro.