The long-term space flight induces a loss of bone density. However, the mechanism has not been well understood, especially about gravity effect on osteoclast. To elucidate the gravitational effect on osteoclasts, we examined the rabbit primary osteoclasts applied to hypergravity and compared the mRNA expression of two kinds of osteoclast marker enzymes, TRAP (tartrate-resistant acid phosphatase) and cathepsin K at the transcription level. Rabbit osteoclasts isolated according to the modified method of Kakudo et al. were cultured on ivory and exposed to 30 x g for 2 hr or 18 hr by placing the culture tubes in a swinging bucket rotor. Results by RT-PCR suggested that hypergravity enhanced the mRNA expression of both enzymes with different manner; the expression of the TRAP showed a smaller increase, that of the cathepsin K showed a non-monotonous time course with maximum hypergravity effect for 2 hr incubation. Moreover we examined the hypergravity effect on actin ring formation in osteoclasts; however, we got no hypergravity effect on the numbers of activated osteoclasts with actin ring formation. These results might suggest that hypergravity has no effect on the number of osteoclasts with resorption activity, but enhances the resorption activity of activated osteoclasts.