To test the hypothesis that an altered gravitational environment induces plastic changes in the vestibulo-cardiovascular reflex, arterial pressure (AP) and hypothalamic glutamate concentration were examined in 2 groups of conscious rats, i.e., a 3-G group and a 1-G group, in which rats were maintained under a 3-G and 1-G environment for 2 weeks, respectively. The vestibulo-cardiovascular reflex was stimulated by a gravitational change induced by a parabolic flight that consisted of 3 phases: "pull-up", during which the G load gradually increased to 2G; a 20s "push-over" into microgravity; and "pull-out", during which the G load increased to 1.8. In the 1-G group, the AP increased by 11.9+/-1.2 mmHg during the pull-up hypergravity period. The AP response was significantly attenuated in the 3-G group (4.0+/-0.8 mmHg). During the push-over microgravity period, the AP decreased from the peak level in the pull-up period and recovered to the pre-parabolic control level (-1.8+/-2.4 mmHg). In rats of the 3-G group, the AP was not altered by push-over microgravity. These AP responses were associated with a significant increase in the glutamate concentration in the hypothalamus (4.4+/-0.7%). The glutamate response was also significantly attenuated in the 3-G group compared with that in the 1-G group. These results indicate that an altered gravitational environment induces plastic alterations in the vestibulo-cardiovascular reflex.