Background. The anti-G suit used in the Gripen fighter is tightly fitted and pre-inflated to 1.3 kPa at 1 Gz. We hypothesized that such pressurization of the lower body causes redistribution of blood from the lower limbs to the thorax with an increased urine production, which in combination with sweating and inadequate intake of liquid might result in dehydration. The aim was to study the effects of wearing an anti-G suit during 8 hrs, with and without fluid intake, on plasma volume and cardiovascular control. Methods. Two experimental series were conducted. In series I, 12 subjects performed an 8-hr simulated mission (8- hrM) at 1 Gz. They were not allowed to eat or drink during the test. Plasma volume, relaxed G-tolerance and orthostatic tolerance (as determined from heart rate and arterial pressure responses) were determined before and after 8-hrM. In series II, 9 subjects performed a similar 8-hrM but were allowed to eat and drink ad libitum. Results. The 8-hrM reduced plasma volume by 7.7 ± 5.9% and 5.1 ± 5.9% in series I and II, respectively. 8-hrM reduced G-tolerance by about 0.2 G in both series. Likewise, 8-hrM reduced orthostatic tolerance in a similar manner in series I and II. Conclusions. That the 8-hrM-induced effects on plasma volume, G- and orthostatic tolerances were not counteracted by fluid intake supports the notion that these effects were due to an anti-G suit induced shift in blood volume to the thorax resulting in increased urine production