N-Desmethylmethsuximide (NDM), the active metabolite of the antiepileptic agent methsuximide, has been analyzed by gas-liquid chromatography and high-performance liquid chromatography (HPLC) in the past. This study compares methods using two commercially available immunoassays for ethosuximide, the enzyme multiplied immunoassay technique (EMIT) and fluorescence polarization immunoassay (FPIA), with an HPLC method for the measurement of NDM concentrations in serum. Within-day precision studies, utilizing low therapeutic (15.0 mg/L) and toxic (45.0 mg/L) NDM concentrations (n = 20), resulted in coefficients of variation (CVs) of 4.6 and 4.2%, respectively, for EMIT and 5.4 and 3.2%, respectively, for FPIA. Day-to-day precision studies (n = 10) resulted in CVs of 7.6 and 5.5%, respectively, for EMIT and 3.5 and 2.4%, respectively, for FPIA. No interference was observed from toxic concentrations of acetaminophen, caffeine, carbamazepine, methsuximide, phenobarbital, phensuximide, phenytoin, primidone, salicylate, and valproic acid in the EMIT and FPIA procedures. There was good linear correlation between EMIT and HPLC NDM determinations of 50 patient samples (r = 0.970; y = 0.96 x + 0.03), and a similar correlation between FPIA and HPLC NDM determinations in 48 patient samples (r = 0.975; y = 0.91 x + 1.24). Using ethosuximide reagents, both EMIT and FPIA systems can be adapted to reliably measure NDM serum concentrations.