The duration of action and hemodynamic mechanisms of isosorbide dinitrate in a sustained release formula (ISDSR) were examined over a 24-hour period. 20 patients with effort angina pectoris were divided randomly into two groups, one group received a single dose of 40 mg ISDSR (at 07.30 h) and the other, two doses of 40 mg ISDSR (at 07.30 and 13.30 h). Every patient had four exercise tests on the first (placebo) day and second (ISDSR) day and a single test on the third (placebo) day. Effort tolerance was improved significantly (p < 0.05) with ISDSR and this lasted for more than 10 h. The double product was unchanged when placebo was compared to ISDSR. Improvement was achieved by a decrease in blood pressure and an increase in heart rate. There were no signs of increased ischemia (ST segment depression) with the prolongation of exercise time. The antianginal and hemodynamic effects of ISDSR were more pronounced in the patients who received two doses of ISDSR. Thus, ISDSR improved effort tolerance for a prolonged period by inducing a sustained decrease of blood pressure and increase in heart rate in patients with effort-induced angina pectoris.