Neuronostatin, a newly identified peptide hormone sharing the same precursor with somatostatin, exerts multiple pharmacologic effects in gastrointestinal tract, hypothalamus and cerebellum. However, the cardiovascular effect of neuronostatin is unknown. The aim of this study was to elucidate the impact of neuronostatin on cardiac contractile function in murine hearts and isolated cardiomyocytes. Short-term exposure of neuronostatin depressed left ventricular developed pressure (LVDP), maximal velocity of pressure development (± dP/dt), and heart rate in Langendorff heart preparation. Consistently, neuronostatin inhibited peak shortening (PS) and maximal velocity of shortening/relengthening (± dL/dt) without affecting time-to-PS (TPS) and time-to-90% relengthening (TR₉₀) in cardiomyocytes. The neuronostatin-elicited cardiomyocyte mechanical responses were mimicked by somatostatin, the other post translational product of preprosomatostatin. Furthermore, the neuronostatin-induced cardiomyocyte mechanical effects were ablated by the protein kinase A inhibitor H89 (1 µM) and the Jun N-terminal kinase (JNK) inhibitor SP600125 (20 µM). The protein kinase C inhibitor chelerythrine (1 µM) failed to alter neuronostatin-induced cardiomyocyte mechanical responses. To the contrary, chelerythrine but not H89 abrogated somatostatin-induced cardiomyocyte contractile responses. Our results also showed enhanced c-fos and c-Jun expression in response to neuronostatin exposure (0.5 to 2 hrs). Taken together, our data suggest that neuronostatin is a peptide hormone with overt cardiac depressant action. The neuronostatin-elicited cardiac contractile response appears to be mediated, at least in part, through a protein kinase A- and/or JNK-dependent mechanism.