Introduction. A recently developed mini ultrasound device for measurement of peripheral tissue thickness is now available for use in clinical practice. Whether this device allows a better guidance of perioperative fluid therapy has to be investigated. Therefore, it is necessary to get basic data on the parameter tissue thickness in otherwise healthy patients during surgery. The aim of the present study was to evaluate differences in tissue thickness change between patients in supine and head down position with a novel handheld ultrasound device during the perioperative course of healthy surgical patients under a standardized fluid regimen. Methods. After obtaining ethics committee approval and informed consent we studied 19 ASA 1-2 female patients undergoing gynecological procedures in supine (SUP, n = 11) or in 30°head down position (HD, n = 8) in general anesthesia. Preoperative NPO status was comparable in both groups. Lactated Ringer's solution (LR) was continuously infused at a rate of 8 ml/kg b.w./h over 90 min and tissue thickness (TT) was determined by ultrasound before induction (t0) and in 30 min intervals (t30, t60, t90) at the forehead. Simultaneously plasma viscosity (PV) was evaluated. Results. Group SUP presented at t0 a forehead TT of 5.3 mm (SD ± 0.5), at t30 TT was unchanged. At t60 mean TT increased significantly to 5.6 mm, (±0.6). At t90 mean TT remained stable at 5.7 mm (±0.5). Group HD presented at t0 a mean TT of 4.6 mm (±0.7), at t30 mean TT was 4.9 mm (±0.7) and at t60 mean TT of the forehead skin was measured as 5.3 mm (±0.6). Significance to t0 was reached at t90 with a mean TT of 5.4 min (±0.7). Group HD showed a steeper increase and a parallel stabilization phase at the end. Differences between t0 and t90 have been significant. Mean PV in the SUP group at t0 (1.361 mPa*s, SD: ±0.045) decreased under the infusion therapy to 1.276 mPa*s (±0.04) at t90. Mean PV in the HD group was determined 1.351 mPa*s (±0.06) at t0 and declined to 1.274 mPa*s (±0.03) at t90. Conclusions. The findings suggest that fluid replacement after an NPO period and the expected changes of forehead TT due to positioning of the patient are detectable by this new ultrasound device.