A large number of studies have been and are being carried out to examine the role of nitric oxide in the hyperdynamic and hypodynamic stages of sepsis. It remains unknown, however, whether adrenomeduilin (ADM), a novel potent vasodilatory peptide, is up-regulated during hyperdynamic sepsis and, if so, whether its production is sustained during hypodynamic sepsis. To determine this, rats were subjected to sepsis by cecal ligation and puncture (CLP), followed by administration of 3 mL/100 g body weight normal saline to these and sham-operated animals. Blood samples were taken at 1, 1.5, 2, 5, and 10 h (2-10 h post-CLP represents the hyperdynamic stage of sepsis) or at 20 and 30 h after CLP (i.e., the hypodynamic stage). Plasma levels of ADM were measured by radioimmunoassay. Adrenomedullin gene expression in various tissues was examined at 2, 10, or 20 h after CLP by reverse transcription-polymerase chain reaction (RT-PCR). The results indicated that plasma levels of ADM did not increase at 1 and 1.5 h after CLP but increased significantly at 2 h after the onset of sepsis. Moreover, circulating ADM increased progressively at 5-20 h and remained elevated at 30 h after CLP. The increased levels of plasma ADM during sepsis were correlated with up-regulation of ADM mRNA in the small intestine, left ventricle, and thoracic aorta. In contrast, ADM gene expression in renal and hepatic tissues was not significantly altered following the onset of sepsis. The association between the up-regulated ADM and the occurrence of hyperdynamic circulation during the early stage of sepsis (both occur at 2 h after CLP) may indicate a possible cause and effect relationship between the two events. Since we have previously shown that ADM-induced vascular relaxation decreased at 20 h after CLP, it appears that the down-regulation of ADM receptors may be responsible for the transition from the hyperdynamic stage to the hypodynamic stage of sepsis.