Behavioral effects of Flutazolam (MS-4101), a new derivative of benzodiazepines, was investigated and compared with effects of diazepam in mice and rats. MS-4101 suppressed hyperemotionality in septal rats, fighting behavior in long-term isolated mice and pentylenetetrazol convulsion and potentiated thiopental sleep. These effects of MS-4101 were the same in potency as those of diazepam. MS-4101 was more potent than diazepam in reducing the spontaneous locomotor activity in the open-field test and potentiating the stimulant effect of methamphetamine on locomotor activity. On the other hand, suppression of hyperemotionality in O. B. rats, potentiation of ethanol-induced anesthesia, prevention of maximal electroshock, prevention of strychnine convulsin and muscle relaxant effect of MS-4101 were less potent than in the case of diazepam. MS-4101 had also an anticonflict effect, which was less potent than that seen with diazepam. Suppression of locomotor activity was potentiated by chronic administration of MS-4101, but disappeared with chronic administration of diazepam. MS-4101 inhibited considerably both scratching and head-twitch induced by mescaline in mice. Scratching was increased with small doses of diazepam and decreased with high doses. Head-twitch was decreased with small doses of diazepam and increased with high doses.