Fourteen men and six women, 48–68 years old, with stable angina and effort-induced ST-segment depression (ST-D) were treated with isosorbide-5-mononitrate (IS-5-MN) 2 × 40 mg/day and/or atenolol (AT) 100 mg/day in a double-blind randomized sequence during two 6-week periods. The patients performed ergometer tests. AT caused more decrease of heart rate at rest and at comparable work-load than IS-5-MN. Blood pressure at rest was in the normal range. Decrease of blood pressure at rest and at effort was similar with both agents. Combined administration of the two drugs was not more effective than monotherapy with AT or IS-5-MN in lowering heart rate and blood pressure. The average ST-D at comparable effort was for placebo 2.3 mm, for IS-5-MN after 2, 4, and 6 weeks, 3, 6, and 12 h, respectively, after medication, 1.4, 1.0, and 1.3 mm, and for AT 1.2, 1.4, and 1.4 mm, respectively. Administration of the drugs together caused additional highly significant reduction of ST-D (0.3–0.9 mm). The results indicate that IS-5-MN and AT have a similar beneficial effect on effort-induced myocardial ischemia, which is enhanced by their combined administration. The drugs alone and in combination are effective for as long as 12 h after administration of IS-5-MN, and 24 h after administration of AT. Moderate signs of tolerance to IS-5-MN were found after 6 weeks of therapy.