Objective:To determine lethal median dose(LD_(50)) and histopalhological toxicity of water extract of Hololhuria atra(H,atra) in mice.Methods:The behavioral changes,mortality and histopathology examination on liver were assessed in mice 14 d after the administration(i.p.) of H.atra water extract.Seven doses 110,20,30,50.100.150 and 200 mg/kg) of H.atra were used.The control group was treated with normal saline,Results:In the acute study in mice,the water extracts of H,atra caused dose-dependent general behavior adverse affects and mortality.The main behavioral sign of toxicity was hypoactivity,noticed immediately after administration of the extract which was more obvious at the higher doses and persisted until death.Mortality increased with increasing doses,the calculated LD_(50)was 41 mg/kg in mice.The liver toxicity was confirmed by histopalhological examination,which indicated the presence of abnormal hepatocytes with a distorted shape and undefined cell lining as well as enlarged nuclei in low doses groups.High doses groups indicated a more prominent distortion of the polyhedral hepatocytes with undefined cell lining,massive cytoplasm,pvknotic.karyorhexis and karyolytie nuclei(necrosis of hepatocytes).Control group showed polyhedral hepatocytes with defined cell lining arranged in cords and normal round nuclei,with granular cytoplasm.Conclusions:Because of the relatively low LD_(50) value in the acute study in mice,it may be concluded that the H.atra water extract is toxic. Objective: To determine lethal median dose (LD_ (50)) and histopalhological toxicity of water extract of Hololhuria atra (H, atra) in mice. Methods: The behavioral changes, mortality and histopathology examination on liver were assessed in mice for 14 days after the administration (ip) of H.atra water extract. Seven doses 110, 20, 30, 50.100.150 and 200 mg / kg) of H.atra were used.The control group was treated with normal saline, Results: In the acute study in mice, the water extracts of H, atra caused dose-dependent general behavior adverse affects and mortality. The main behavioral sign of toxicity was hypoactivity, noticed immediately after administration of the extract which was more obvious at the higher doses and persisted until death. Mortality increased with increasing doses, the calculated LD 50 (50) was 41 mg / kg in mice. The liver was washes by histopalhological examination, which showed the presence of abnormal hepatocytes with a distorted shape and undefined cell lining as well as enlarged as n uclei in low doses groups. High doses groups indicated a more prominent distortion of the polyhedral hepatocytes with undefined cell lining, massive cytoplasm, pvknotic. karyorhexis and karyolytie nuclei (necrosis of hepatocytes). Control group showed polyhedral hepatocytes with defined cell lining arranged in cords and normal round nuclei, with granular cytoplasm.Conclusions: Because of the relatively low LD_ (50) value in the acute study in mice, it may be concluded that the the H.atra water extract is toxic.