The potential of bioelectrical impedance spectroscopy (BIS) for assessing nutritional status in space flight was tested in two head-down-tilt bed-rest studies. BIS-predicted extracellular (ECW), intracellular (ICW), and total body (TBW) water measured using knee-elbow electrode placement were compared to deuterium and bromide dilution (DIL) volumes in healthy, 19-45 yo subjects. BIS was accurate during 44h of head-down-tilt with mean differences (BIS-DIL) ranging from 0-0.1 kg for ECW, 0.3-0.5 for ICW, and 0.4-0.6 kg for TBW (N=28). At the 44 h time point, BIS followed the within-individual change in body water compartments with a relative prediction error (SEE/baseline volume) of 2.0 to 3.6% of water space. In the second study, BIS did not detect an acute decrease (-1.41±0.91 kg) in ICW secondary to 48 h of a protein free, 800kcal/d diet (N=18). The insensitivity to ICW losses may be because they were predominantly (65%) localized to the trunk and/or a general failure of BIS to measure ICW independently of ECW and TBW. BIS may have potential for measuring nutritional status during space flight, but its limitations in precision and insensitivity to acute ICW changes warrants further validation studies.