Serum-deprived mouse osteoblastic cells (MC3T3-E1a) were centrifuged under a regime designed to simulate a space shuttle launch (maximum of 3g). Messenger RNA levels for eight genes involved in bone growth and maintenance were determined using RT-PCR. Following 30 min of centrifugation, mRNA level for early response genec-foswas significantly increased 89% (P< 0.05). Thec-fosinduction was transient and returned to control levels after 3 h. The mRNA level for the mineralization marker gene osteocalcin was significantly decreased to 44% of control level (P< 0.005) 3 h after centrifugation. No changes in mRNA levels were detected forc-myc,TGFβ1, TGFβ2, cyclophilin A, or actin. No basal mRNA level for TGFβ3 was detected. In addition, no change in the steady-state synthesis of prostaglandin E 2 was detected, possibly due to lack of lipid substrates in serum-deprived cells, suggesting that the increase inc-fosmRNA in response to gravitational loading is a result of mechanical stimulation. These results indicate that a small magnitude mechanical loading, such as that experienced during a shuttle launch, can alter mRNA levels in quiescent osteoblastic cells.