Microgravity resulting from free drop elicits a pressor response that involves both vestibular and nonvestibular pathways. In rats reared under a 3G environment for 2 weeks, plastic alterations in both vestibular- and nonvestibular-mediated responses are induced; specifically, the pressor responses involving both pathways are reduced [C. Abe, K. Tanaka, C. Awazu, H. Chen, H. Morita, Plastic alteration of vestibulo-cardiovascular reflex induced by 2 weeks of 3-G load in conscious rats, Exp. Brain Res. 181 (2007) 639-646]. It is currently unknown whether plastic alterations in the nonvestibular system depend on the vestibular system. To examine this topic, the pressor response to free drop was compared between rats with and without vestibular lesion (VL) reared under 1G or 3G environments. The pressor response to free drop was 34+/-3mmHg in vestibular intact rats reared under 1G, and was significantly attenuated in rats reared under a 3G environment for 2 weeks (13+/-3mmHg); however, the pressor response was similar between VL-1G (18+/-3mmHg) and VL-3G (19+/-3mmHg) rats. Therefore, the 3G environment induced plastic alterations in the pressor response to free drop mediated by both the vestibular and nonvestibular systems, and the vestibular system is indispensable for induction of the plastic alteration of the nonvestibular-meidated pressor response to free drop.