A marked increase in adrenomedullin (AM) concentration was observed in rat plasma collected after intravenous infusion of lipopolysaccharide (LPS). Plasma immunoreactive (ir-) AM concentration increased already after 1 h exposure to 5 mg/kg of LPS and was elevated to 49.7 +/- 2.3 fmol/ml (mean +/- SEM) at 3 h after injection in contrast to 2.7 +/- 0.3 fmol/ml in saline-injected control rats. Plasma ir-AM level increased in a dose-dependent manner in a range of 0.008 to 5.0 mg/kg of LPS. AM gene transcription in LPS-injected rats was augmented more than 3-fold in ileum, liver, lung, aorta, and 2.4-fold even in skeletal muscle, in which AM was not thought to be produced in myocytes. These results, along with our recent data that AM production in cultured vascular smooth muscle cells (VSMCs) is stimulated with LPS, indicate that AM production is highly augmented in blood vessel, lung and intestine by administration of LPS. Since VSMCs express AM-specific receptors, secreted AM is deduced to actually exert a vasorelaxant effect especially in the endotoxin shock.