BACKGROUND: Though the proper administration of midazolam in the geriatric patients under the spinal anethesia reduce the stress, the anxiety and the agitation during surgery, it can cause the hypoxemia owing to decreasing their ventilatory function, and the aim of this study was to evaluate the dose response of midazolam to determine the degree of hypoxemia, the change of cardiovascular functions after the intravenous administration of midazolam in geriatric patients during the period of TURP under the spinal anesthesia. METHODS: On thirty two geriatric male patients(> or =65 yrs), the changes of O2 saturation in pulse oximetry, systolic and diastolic pressure in NIBP, heart rate in EKG before and after the administration of midazolam were studied during perioperative period of TURP under the spinal anesthesia in randomized method, and they were allocated randomly to four groups to receive only normal saline (group 1), midazolam 0.01 mg/kg(group 2), midazolam 0.02 mg/kg(group 3), midazolam 0.03 mg/kg (group 4) in normal saline 3ml, respectively. RESULTS: The results were that the desaturation between 85% and 90% in SpO2 occurred in three patients(one patient in group 3, two patients in group 4), that the desaturation below 85% in SpO2 occurred in three patients (group 4), immediately 100% oxygen was administered to the patients by mask, that the occurrances of the desaturation below 90% in SpO2 appeared about 4-7minutes after the administration of midazolam and there was no statistically significant changes in cardiovascular function in all groups. CONCLUSIONS: It is concluded that the intravenous administration of midazolam more than 0.02 mg/kg can cause hypoxemia in geriatric patients under the spinal anesthesia and the close observation and monitoring must be needed during sedative period.