Each of 13 patients with angina had either chewable isosorbide dinitrate, nitroglycerin (sublingual therapy), or placebo administered on each of three different days when mild anginal pain had been induced by walking on a treadmill. Both therapy with isosorbide dinitrate and therapy with nitroglycerin were effective in bringing about complete relief of the angina in less than ten minutes of uninterrupted continuous walking on the treadmill in 11 of the 13 subjects, while administration of placebo afforded complete relief in none. The duration of action of the drug was determined by following the first effort (during which the tested drug was given) by successive ten-minute walks at the same workload that first induced anginal pain. Half-hour resting periods separated the repeated periods of exercise, and the duration of action was taken to be the time from administration of the agent to the return of angina on one of the repetitive efforts. No prolonged protection was afforded by administration of the placebo. Nitroglycerin protected for slightly longer than one hour, while isosorbide dinitrate protected for 2 1/2 to 3 hours.