For many years it has been known that the dosage of thiopental required to induce anesthesia depends on the age of the patient, but this information ha resulted from clinical experience with the drugs rather than from planned study. In our study, to elucidate the influence of age on the size of the sleeping dosage of thiopental, 144 patients who underwent minor orthopedic and gynecological operations were studied. This patients had no evidence of a disease other than that scheduled for operation and were within normal values in hematologic examination, liver function and kidney function. Using a simple "yes" or "no" verbal command response, as sleeping response, to a single bolus of thiopental in mg/kg body weight, we have attemted to minimize uncontroliable factors such as cerebral perfusion, circulation time and plasma protein binding which would alter response to thiopental infusions continued to the end points. The results were as follows: 1) There was no statistical difference in verbal command response to thiopental according to age in age groups below 59 years. 2) Compared with age groups below 59 years, 60~79yrs, group failed to respond to the verbal command in 33% by 2.2mg/kg and 100% by above 2.6mg/kg.(p<0.05) 3) The mean time for loss of verbal command response after thiopental injection was 27.4+/-10.3 seconds and 80.4% of theses patients lost lid reflex with a mean time of 41.7+/-9.1 seconds after thiopental injection. 4) All groups showed statistically non-significant alterations of systolic blood pressure and heart rate after injection of thiopental.