We aimed to investigate median lethal dose (LD(50)) and hypoglycemic effect of Thymus fallax F. leaves essential oil (TF) on healthy and diabetic mice. TF, glibenclamide (as a reference group) and isotonic saline solution (ISS) (control group) were administrated to the healthy and alloxan-induced diabetic mice. Blood samples were drawn from the vena coccygea of mice before treatment and in the 1st, 2nd, 3rd, 4th and 24th hour for measurement of glucose levels. When compared with ISS group TF decreased blood glucose values significantly in diabetic mice in the second hour. Glibenclamide decreased blood glucose values significantly in the 1st, 2nd, 4th and 24th hour. There was no significant difference of blood glucose values among groups of healthy mice. The median lethal dose (LD(50)) of the TF was determined at 0.741 mL/kg.