Objectives : The object of this study was to obtain accurate information (single oral dose toxicity) of Lonicerae Flos (LF; Dried flower bud parts of Lonicera japonica Thunb (Caprifoliaceae)), which has traditionally been used in Korean medicine for treating various inflammatory diseases. Methods : In order to observe the 50% lethal dose (LD 50), approximate lethal dosage (ALD) and target organs, test articles were once orally administered to female and male ICR mice at dose levels of 2,000, 1,000, 500 and 0 (control) mg/kg (body weight.). The mortality and changes on body weight, clinical signs and gross observation were monitored for 14 days after single oral treatment of LF aqueous extracts with organ weights and histopathological observations of 12 types of principle organs. Results : 1. After single oral treatment of LF aqueous extracts, we could not find any mortality and toxicological evidences up to 2,000 mg/kg treated group, the limited dosages in rodents at body and organ weights, clinical signs, gross and histopathological observations. 2. Slight diarrhea was detected in most mice treated with 2,000 mg/kg of LF aqueous extracts and male mice of LF aqueous extracts 1,000 mg/kg within 2 days after end of treatment, respectively. Conclusion : The results obtained in this study suggest that the LD 50 and ALD of LF aqueous extracts in both female and male mice after single oral treatment were considered as over 2,000 mg/kg because no mortalities were detected up to 2000 mg/kg, the highest dose recommended by KFDA and OECD. However, we also observed the possibility of digestive disorders like diarrhea when over 1,000 mg/kg of LF aqueous extracts were administered in the present study.