Adrenomedullin (ADM) is an endogenous peptide with favourable hemodynamic effects in vivo. In this study we characterized the direct functional effects of ADM in isolated preparations from human atria and ventricles. In electrically stimulated human nonfailing right atrial trabeculae ADM (0.0001 to 1 µmol/L) increased force of contraction in a concentration dependent manner with a maximal increase by 35 ± 8 % (at 1 µmol/L; p<0.05). The positive inotropic effect was accompanied by a disproportionate increase in calcium transients assessed by aequorin light emission (by 76 ± 20 %; force/light ratio (F/L) 0.58 ± 0.15). In contrast, elevation of extracellular calcium ([Ca²⁺]₀; from 2.5 to 3.2 mmol/L) proportionally increased force and aequorin light emission (F/L 1.0 ± 0.1; p < 0.05 vs. ADM). Consistent with a cAMP- (cyclic adenosine monophosphate) dependent mechanism ADM (1 µmol/L) increased atrial cAMP levels by 90 ± 12%, and its inotropic effects could be blocked by the protein kinase A (PKA) inhibitor H89. ADM also exerted positive inotropic effects in failing atrial myocardium and in nonfailing and failing ventricular myocardium. The inotropic response was significantly weaker in ventricular vs. atrial myocardium and in failing vs. nonfailing myocardium. In conclusion, ADM exerts Ca²⁺-dependent positive inotropic effects in human atrial and less pronounced in ventricular myocardium. The inotropic effects are related to increased cAMP-levels and stimulation of PKA. In heart failure the responsiveness to ADM is reduced in atria and ventricles.