Conditions of disuse such as bed rest, space flight, and immobilization result in decreased mechanical loading of bone, which is associated with reduced bone mineral density and increased fracture risk. Mechanisms involved in this process are not well understood except the suppression of osteoblast function. To investigate the effect of simulated weightlessness on mRNA level of extracellular matrix proteins, osteoblasts were rotated in horizontal plane as a model of simulated microgravity. Primordial osteoblasts of rats were grown for 2 d and then rotated for 24, 48 and 72 h, respectively. After isolating total RNA in cells, reverse transcription polymerase chain reaction (PT-PCR) was made to examine the mRNA level of osteopontin (OPN) and osteonectin (ON). Meanwhile, the levels of alkaline phosphatase (ALP) and osteocalcin (BGP) in the cultured medium were measured to evaluate the calcific function of cell. The expression of OPN and ON mRNA fell significantly after rotating for 24, 48 and 72 h, respectively. The contents of BGP descended significantly, meanwhile, the activity of ALP also showed a degressive tendency. Horizontal rotation decreased the expression of ON and OPN as well as diminished the secretion of BGP and ALP, which affected the calcific function of osteoblast. The results obtained suggest that depression of extracellular matrix proteins expression plays a key role in bone loss during weightlessness.