β-Histine-HCl has been shown to be effective in modifying the size of developing myocardial infarcts. In the present study the hypothesis that β-hιstine increases the flow of blood through collateral channels and thus supplies blood to ligated areas of the myocardium was investigated in the dog. The methods used were measurement of retrograde coronary blood flow and angiography after coronary artery ligation. β-Histine administration for 6 h increased retrograde blood flow 68.2–91.0% over controls. Coronary angiography demonstrated the existence of collateral channels 200–400 µm in diameter within the myocardium after ligation and 4 h of β-histine admin