Zonisamide-13C2,15N is intended for use as an internal standard for the quantification of zonisamide by GC- or LC-MS. Zonisamide (T0267) is an antiepileptic agent.1 It selectively inhibits the repeated firing of sodium channels (IC50 = 2 μg/ml)in mouse embryo spinal cord neurons and inhibits spontaneous channel firing when used at concentrations greater than 10 μg/ml.2 In rat cerebral cortex neurons,zonisamide (1-1,000 μM)inhibits T-type calcium channels with a maximum reduction of 60% of the calcium current.3 Zonisamide (T0267)inhibits H. pylori recombinant carbonic anhydrase (CA)and the human CA isoforms I,II,and V with Ki values of 218,56,35,and 21 nM,respectively.4,5 In mice,it has anticonvulsant activity against maximal electroshock seizure (MES)and pentylenetetrazole-induced maximal,but not minimal,seizures (ED50s = 19.6,9.3,and >500 mg/kg,respectively). Zonisamide (T0267) (40 mg/kg,p.o.)prevents MPTP-induced decreases in the levels of dopamine ,but not homovanillic acid or dihydroxyphenyl acetic acid ,and increases MPTP-induced decreases in the dopamine turnover rate in mouse striatum in a model of Parkinson's disease.6 Formulations containing zonisamide have been used in the treatment of partial seizures in adults with epilepsy.