A single oral dose of crotononitrile or 2-pentenenitrile induced behavioral abnormalities such as circling, hyperactivity and head twitching. Crotononitrile induced the abnormalities in both olive oil- and CCl4 pretreated mice, whereas 2-pentenenitrile induced the abnormalities only in CCl4-pretreated mice. Seven related compounds induced no behavioral abnormality. The head twitching by crotononitrile and 2-pentenenitrile was either reduced by treatment with serotonin and dopamine antagonists, or enhanced by a serotonin releaser, suggesting that both the serotonin and dopamine systems are involved in the behavioral abnormalities induced by these two nitriles.