Background : Head-down suspension (HOS) of rats has evolved as a useful model for the simulation of a microgravity environment. Atrial natriuretic peptides (ANP) and brain natriuretic peptide (BNP) have been considered to comprise the cardiac natriuretic peptide family responsible for body fluid homeostasis and blood pressure control. However, little is known about the long-term responses and the simultaneous observations of both ANP and BNP following HOS. Therefore, this study was aimed to characterize the long-term adaptations of cardiac ANP and BNP 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 cardiac synthesis of natriuretic peptides as a expression of ANP and BNP mRNA, The expressions of natriuretic peptide mRNA were measured by reverse transcription- polymerase chain reaction with [³²P]-dCTP following 4 weeks of HOS in the each cardiac chamber of control and HOS rats. Result : After 4 weeks of HOS, the expressions of ANP mRNA were decreased in the both right (P<0.05) and left atria. The ANP mRNA in the ventricles did not show significant changes compared with control values. In contrast with ANP, BNP mRNA was only decreased in the left atrium (P<0.05) and revealed the non-significant decreasing trend in the right ventricle. There were no remarkable changes of BNP mRNA expression compared with control rats in the both right atrium and left ventricle. Conclusion : Collectively, these results represent that the cardiac syntheses of natriuretic peptides following 4 weeks of HOS are attenuated to prevent any decrement of body fluid as a long-term adaptive response to simulated microgravity and ANP and BNP genes have the differential expression in the each chamber of the heart.