Single dose toxicity studies of landiolol hydrochloride (ONO-1101), a novel ultra short acting beta-blocker, were conducted in Sprague-Dawley (SD) rats and beagle dogs. ONO-1101 was administered intravenously at a dose level of 37.5, 75, 150 or 300 mg/kg to rats of both sexes and 25, 50 or 100 mg/kg to male dogs. In the rat study, 5/6 males in the 150 mg/kg group and all animals in the 300 mg/kg group died during or right after administration. Survivors in the 150 mg/kg group showed temporal hypoactivity, bradypnea, dyspnea, tremor, loss of righting reflex and reddish lacrimation up to 5 min after injection. One male in the 150 mg/kg group had a tendency of suppression on body weight gain. No effects on clinical signs and body weight gain were seen in the 75 mg/kg group or lower. Necropsy findings showed only red tear in the majority of the decedents. In the dog study, all animals died within 6 min after administration in the 100 mg/kg group, showed ataxic gait, rolling and tachypnea followed by bradypnea and gasping/apnea. Incontinence of urine, defecation and vocalization were also seen in each one of two animals before death. Temporal hypoactivity was seen 1 min after administration in the 50 mg/kg group. No clinical signs were seen in the 25 mg/kg group. ONO-1101 did not affect bodyweight or food consumption. Necropsy findings of the decedents showed no abnormalities. It is indicated that the minimum lethal doses are 150 mg/kg in rats and 100 mg/kg in dogs.