The aim of the present work is to confirm the presence of MDF (myocardial depressant factor), which has long been postulated to be one of the cardiotoxic substances in the shock state. Twenty male mongrel dogs were divided into two groups, a hemorrhagic shock group (n = 10) and an endotoxic shock group (n = 10). Blood samples were obtained from each animal at 1, 2, 4, and 7 h after hypotensive events occurred. Inotropic properties of the plasma samples were evaluated by the isometric contraction of a cat papillary muscle preparation, and chromatographic analysis was performed on the peptides in the plasma. Developed tension of the muscle was increased significantly by changing the bathing medium from Krebs-Henseleit solution to plasma obtained 1 to 4 h after the onset of hemorrhagic and endotoxin induced hypotension. The positive inotropic change was associated with a significant increase in plasma epinephrine concentration. None of these plasma samples possessed a negative inotropic effect (i.e., the property of MDF activity). The elution profile by gel column chromatography of samples obtained from shocked animals was almost identical to that recognized as MDF. However, the presence of MDF was not confirmed by column parameters and color development by the ninhidrin reaction. In conclusion, we found no evidence to support the presence of cardiotoxic peptide in plasma of shocked animals.