In one cosmonaut, we investigated lower body negative pressure (LBNP)-induced capillary fluid shifts which we hypothesized would be changed by microgravity (micrograms) adaptation. Sound pulse velocity (SV) was determined in whole blood and plasma samples, using a new method that could detect 0.1% protein concentration changes. Experiments were performed 3 months preflight (supine), during space flight (6th day in orbit), and postflight (supine; 4th day after landing). Antecubital blood was taken at the beginning (3 min: time a) and after shut-down (+2 min: time b) of 40 min LBNP (-15/-30/-35 mm Hg for 15/15/10 min, respectively), since in control experiments with multiple sampling on Earth, the largest difference (increase) between blood and plasma SV was observed between a and b. Our cosmonaut had a 1.6 m/s increase in blood sound pulse velocity (BSV) preflight and a 4.0 m/s increase postflight, whereas BSV stayed unchanged in flight. Plasma sound pulse velocity (PSV) increased 1.2 m/s preflight and 1.7 m/s postflight, whereas PSV did not rise (-0.4 m/s) in flight. This would indicate profoundly altered LBNP-induced fluid dynamics in flight, compared to control (1-g) conditions. On the 4th day postflight, blood and plasma sound velocity increased more with LBNP than preflight, indicating greater hemoconcentration than under control conditions. In summary, the data suggest: 1) altered fluid shifts between blood and interstitial compartments during LBNP with 6 d adaptation to microgravity; and 2) increased hemoconcentration during LBNP early after a 10-d spaceflight.(ABSTRACT TRUNCATED AT 250 WORDS)