Background : Head-down suspension (HOS) of rats has been used as a model for simulation of a microgravity environment. C-type natriuretic peptides (CNP) and atrial natriuretic peptide (ANP) are produced in the central nervous system, especially in hypothalamus, to complement their peripheral natriuretic effects. Therefore, this study investigated the changes in the central adaptations of hypothalamic ANP and CNP syntheses to 4 weeks of HOS in rats. Methods : Unanesthetized, unrestrained, male Sprague-Dawley rats were subjected to either a horizontal position (control rats) or a -45° head-down tilt using the tail-traction technique (HOS rats). We determined the hypothalamic syntheses of natriuretic peptides as an expression of ANP and CNP mRNA. The expression of natriuretic peptide mRNA was measured by reverse transcription-polymerase chain reaction with [32P]-dCTP following 4 weeks of HOS in the hypothalamus of control and HOS rats. Results : After 4 weeks of HOS, the expression of ANP mRNA showed a decreasing trend in the hypothalamus of HOS rats. In contrast with ANP, CNP mRNA expression was significantly (P<0.01) increased in the hypothalamus of HOS rats. There were different changes in the hypothalamic CNP and ANP mRNA expressions of HOS rats compared with that of the control rats. Conclusion : These results represent that the hypothalamic syntheses of natriuretic peptides are differently responded and the role of CNP is augmented to compensate for the decrement of ANP action in the central nervous system following 4 weeks of HOS.