Mannitol is the most commonly used hyperosmotic agent in neurosurgery. Being an agent that increases intravascular volume by withdrawing water from the brain, it may cause significant changes in stroke volume (SV), cardiac output (CO), systemic vascular resistance and blood pressure. In this study, we monitored the hemodynamic changes in response to a single dose of mannitol by using a noninvasive CO monitorbased on the thoracic electrical bioimpedance technique, in patients undergoing craniotomy. Eleven adult patients undergoing elective craniotomy received mannitol 1.0 g/kg 15 minutes before dural opening. The following hemodynamic variables were recorded: heart rate, systolic blood pressure, diastolic blood pressure, SV, CO, and cardiac index. The measurements were made before the administration of mannitol, at 1, 2, 5, 10, 15, 20, 25, 30, 35, 40, and 45 minutes after the termination of the mannitol infusion. Urine output was measured at 10, 20, 30, 40, 50, 60, 90, and 120 minutes after termination of the mannitol infusion. Heart rate values from 25 to 45 minutes were significantly lower compared with the premannitol values (P<0.05). All the postmannitol systolic blood pressure values were significantly lower than the premannitol value (P<0.05). SV increased significantly for 15 minutes after administration of mannitol (P<0.05). SV at 45 minutes was significantly lower than that from 1 to 30 minutes (P<0.05). Cardiac index also showed a similar change with a significant increase at 1 to 10 minutes and a decrease at 40 to 45 minutes compared with 1 to 15 minutes.The rate of urine secretion was higher during the first 10 minutes (40±15 mL/kg/ h) than during the rest of the study period. The overall fluid balance at the end of 120 minutes was −370±987 mL. In this study using noninvasive measurement of CO by thoracic bioimpedance plethysmography during craniotomy, a single bolus dose of mannitol 1.0 g/kg caused a significant but short duration changes in the hemodynamic variables. The changes in SV, and CO, lasted for only 15 minutes after the infusion.