Nine volunteers, aged 27 to 41 years, were exposed to 370-day head-down tilting during which their tibia bones were examined by measuring acoustic parameters with the aid of exponential point concentrators located at a 15 mm distance. Although the bone acoustic profile showed individual variations, the decrease of the average ultrasound velocity in the middle part and the gradient of the ultrasound velocity in the distal part of the diaphysis occurred primarily in the first half of the bed rest study, thus reflecting changes in bone adaptation to mechanical loads (redistribution of elastic properties along the bone). The acoustic parameters measured noticeably responded to countermeasures used (or not used) during bed rest (exercise) and therefore can help obtain better prognostic information about bone is compared to the data accumulated by means of monophoton absorptiometry (mineral density measurement). This method can be well used for examining long bones in extended space flight.