Objectives : The object of this study was to obtain acute toxicity information (single oral dose toxicity) of Armeniacae Semen (AS), a medicinal herb used for treating constipation and various respiratory diseases, in rats. Methods : In order to observe the $LD_{50}$ (50% lethal dose), approximate lethal dosage (ALD) and target organs, AS aqueous extracts were orally administered once to female and male Sprague Dawley rats at dose levels of 2,000, 1,000, 500 and 0 (control) mg/kg (body weight). The mortality, changes in the body weight, clinical signs and gross observation were monitored for 14 days after single oral treatment of AS extracts, and the organ weights and histopathological findings of principle organs were observed after sacrifice. Results : After single oral treatment of AS 2,000 and 1,000 mg/kg, all (5/5; 100%) female and male rats died within 30 minutes after treatment, while no mortalities were observed in the female and male rats treated with 500 mg/kg of AS extract. Therefore, $LD_{50}$ in female and male rats was calculated as 741.95 mg/kg. Seizure, loss of locomotion, salivation, increases of respiration and heart-beat were observed after AS extract treatment, which were observed in all rats including the lowest dosage group, 500 mg/kg in the present study. In addition, lung congestion was visible in all mortal rats with AS 2,000 and 1,000 mg/kg, respectively. Conclusions : The results obtained in this study suggest that AS extract ranges in Class III, because the $LD_{50}$ and ALD in both female and male rats were calculated as 741.95 mg/kg and 500~1,000 mg/kg, respectively. However, AS extract should be carefully treated at clinical applications, because salivation, increase of respiration and heart-beat were also observed in the lowest dosage group, 500 mg/kg in the present study.