The use of bioelectrical impedence (BI) measurement to assess body composition has recently attracted the attention of anesthesiologists. Analysis of BI provides a non-invasive method to quantify fluid distribution in different body compartments. This study was designed to assess whether BI analysis reflects fluid depletion in neurosurgical patients with moderate blood loss. Six adult male patients scheduled for elective craniotomy under general anesthesia were studied. Exclusion criteria included patients with cardio-respiratory disease. BI analysis was performed at three stages, A, day before operation, B, during surgery and C, on the first postoperative day. Total body resistivity was measured by BI analysis with a four-terminal portable impedence analyzer. At each frequency, impedence was calculated as resistance (Rx)2 + reactance (Rc)2. The mean values of total body water (TBW) at stages A, B and C were 39.8 L (range: 33.1-46.7 L), 43.2 L (range: 33.1-66.2 L) and 36.8 L (range: 22.4-36.3 L) respectively with significant differences (P<0.05). The impedence at the three frequencies during stages A, B and C showed significant differences (P<0.05). In conclusion, we have found that in male neurosurgical patients multiple frequency BI measurements has reflected fluid balance perioperatively. Whether this observation remains true for other surgical procedures with massive blood loss, yet to be further investigated.