The aim of this study was to determine the adaptation of atrial natriuretic peptide(ANP) to simulated weightlessness. The level of irnmunoreactive plasma ANP level and the expression of right atrial ANP mRNA were assessed in male Sparague-Dawley rats during 2 weeks of head-down suspension(HDS). The rats were placed in a -45°anti-orthostatic position. The plasma ANP level was investigated by radioirnmunoassay and ANP mRNA was expressed by Northern blot analysis. The changes in daily water intake, body weight and arterial hematocrit did not show the statistical significances during HDS. The plasma ANP level slightly increased after 1 day of HDS, the response was trensient and then decreased below the control levels. The expression of the ANP mRNA increased after 1 day, the peak value occurred at 3 days of HDS and thereafter ANP mRNA showed the similar patterns compared to the changes of plasma level. On the basis of these findings, we conclude that the initial adaptation of ANP to increased central blood volume in rats occurs between 3 and 7 days of HDS, and the ANP acts an important role during regulatory process to central hypervolemia at both synthetic and secretory levels within the early stage of HDS.