Summary(1) Threshold convulsant dosages to rapid intravenous injection of semicarbazide were significantly lower in chronically epileptic monkeys than in normal unoperated controls. (2) Clinical seizures following intravenous injection of threshold convulsant doses of semicarbazide occurred after a latent period of 2 to 3 1/2 hours. With suprathreshold doses the latent period was diminished greatly. (3) Use of a test dose of 30 to 40 mg/kg of semicarbazide by rapid intravenous injection is suggested to distinguish epileptic from non-epileptic monkeys since clinical convulsions should be produced within 3 to 4 hours in epileptic monkeys and not in non-epileptic normals.