A gas-chromatographic analysis for the succinimide anticonvulsant drugs--ethosuximide, methsuximide, and phensuximide--in 1.0 ml of serum was modified to improve its reliability, speed, and precision. A separate procedure for 10-100 mul of serum was also developed. Neither method requires an initial preparation of derivatives. The working range of each method is about 10-100 mg/liter for the macro-method, 2-100 mg/liter for the micro-method. The methsuximide metabolite, N-desmethylmethsuximide, is included in both methods. Concentrations of N-desmethylmethsuximide in the blood of two patients with petit mal epilepsy are reported.