Altered environmental gravity, including both hypo- and hypergravity, may result in space adaptation syndrome. To explore the characteristics of this adaptive plasticity, the expression of immediate early gene c-fos mRNA in the vestibular related tissues following an exposure to hypergravity stimulus was determined in rats. The animals were subjected to a force of 2 g (twice earth's gravity) for 1, 3, or 12 h, and were examined poststimulus at 0, 2, 6, 12, and 24 h. RT-PCR (reverse transcription polymerase chain reaction) and real-time quantitative RT-PCR were adopted to analyze temporal changes in the expression of c-fos mRNA. The hypergravity stimulus increased the expression of c-fos mRNA in the vestibular ganglion, medial vestibular nucleus, inferior vestibular nucleus, hippocampus, cerebellum, and cortex. The peak expression occurred at 0 h poststimulation in animals stimulated with hypergravity for 1 h, and at 6 h poststimulus in those stimulated for 3 h. In contrast, those stimulated for 12 h exhibited dual peaks at 0 and 12 h poststimulus. Bilateral labyrinthectomy markedly attenuated the degree of c-fos mRNA expression. Glutamate receptor antagonist also dramatically attenuated the degree of c-fos mRNA expression. These results indicate that expression of c-fos mRNA in response to hypergravity occurs in the vestibular related tissues of the central nervous system, in which peripheral vestibular receptors and glutamate receptors play an important role. The temporal pattern of c-fos mRNA expression depended on the duration of the hypergravity stimulus.