Objective: The lethal dose 50 (LD50) of lorazepam in albino mice (Musmusculus), swiss ICR strain, was determined as a first step in the study of the conjoint toxicity of admixtures of scopolamine and benzodiazepines (new burundanga). Method: Sixty adult male mice were randomly assigned to five experimental groups and to a control one. The dose of lorazepam administered intraperitoneally to each group was: group 1,10 mg kg_1; group 11,20 mg kgI; group 111,40mg kgL; group IV,80mgkg'; group V,160mg kg-l.The control group received only the vehicle solution. Mortality was recorded during 15 days after injection. Necropsies were performed to all the mice dead during the assay and to the survivors. Data were processed using probit analysis and survival analysis. Results: Estimated LD50 were 90.71 mg kg- 1, with 95% confidence range of 65,02to 150,13mg kg- 1.Deaths occurred within the first six days after injection of doses higher than 80 mg kg:', mostly during the first 48 hours. Conclusions: The estimated LD50 of lorazepam in this experiment almost doubles the reported one, this finding suggests a higher resistance of the mice strain used in this experiment The critic period for lorazepam poisoning spans the first 48 hours.