Microgravity in space has been reported to enhance bone resorption in vitro and in vivo, however, the gravity effect on bone remodeling is not well understood. In par-ticular, there are only a few reports about gravity effect on osteoclasts. In the present study, we examined the hypergravity effect on mRNA expressions of osteoclast marker enzymes such as TRAP (tartrate-resistant acid phosphatase) and cathepsin K. Osteoclasts were exposed to 30 x g for 2 hr or 18 hr. Results suggested that hypergravity enhanced the mRNA expression of both enzymes with different manner; the expression of the TRAP showed a slight increase, that of the cathepsin K showed a non-monotonous time course with maximum hypergravity effect for short time (2 hr) incubation.