Four drug-metabolizing activities in Wistar rat and Mongolian gerbil liver microsomes were investigated to clarify the biochemical basis of convulsions. No significant associations were observed between the susceptibility to the tonic-clonic seizures in Mongolian gerbils and the drug-metabolizing enzymatic activities of p-nitroanisole O-demethylase, aminopyrine demethylase and benzo[a]pyrene-3-monooxygenase. However, a significant association was observed with the debrisoquine 4-monooxygenase (cytochrome P-450db1-linked monooxygenase system) activity, which is known to metabolize exogenous and endogenous parkinsonism-inducing neurotoxins such as MPTP and TIQ, and to activate carcinogens during the detoxication of xenobiotics. The mean activity of debrisoquine 4-monooxygenase in the seizure-sensitive group (male, 380; female, 350 pmol/h/mg protein) remained significantly higher (about 4-5 times) than that in the seizure-resistant group (male, 90; female, 80 pmol/h/mg protein). More Mongolian gerbils were extensive metabolizers of debrisoquine in the seizure-sensitive group than in the seizure-resistant group. These results suggest a possibility that the cytochrome P-450db1-linked monooxygenase system may activate the potential neurotoxins which are associated with tonic-clonic seizures in the Mongolian gerbil.