Effects of 3-acetoxy-2,3-dihydro-5[2-(dimethyl-amino)ethyl]-2-(p-methoxyphenyl)-1,5-benzothiazepin-4 (5H)-one hydrochloride (diltiazem) on coronary circulation of the heart with experimental coronary sclerosis induced by i.v. allylamine (Group A), and on regional myocardial blood flow in acutely-induced ischemic myocardium (Group S) were studied in anesthetized open-chest dogs. Regional myocardial blood flow was continuously measured with heated cross thermocouple method and following results were obtained: 1. In Group A coronary blood flow and coronary flow resistance remained unchanged essentially after the injection of diltiazem, whereas coronary blood flow markedly rose and coronary flow resistance decreased in the normal heart. 2. Local blood flow in both of the inner and outer thirds of the ischemic myocardium was not affected by diltiazem in Group S. In the normal myocardium, however, it increased definitely. 3. From these findings it is concluded that the clinical effectiveness of diltiazem is independent of its vasodilator properties.