Threshold convulsant dosages to rapid intravenous injection of semicarbazide were significantly lower in 5 chronically epileptic monkeys than in 3 normal unoperated controls. Clinical seizures following intravenous injection of threshold convulsant doses of semicarbazide occurred after a latent period of 2 to 3.5 hours. With suprathreshold doses the latent period was diminished greatly. 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 3-4 hours in epileptic monkeys and not in non-epileptic normals.