Nitroglycerin 0.5 mg sublingually was tested in two exercise protocols in order to determine the time of onset of the anti-anginal action. When patients stopped exertion after nitroglycerin administered at the time of moderate chest pain and one minute before stopping exercise, no anti-anginal or anti-ischaemic effect was seen compared with a placebo-medicated test given double-blindly. When the patients continued exertion after nitroglycerin administered at the onset of chest pain, a significant decrease in chest pain intensity and an improvement in ST-segment depression was seen. It is concluded that administration of a quick-acting anti-anginal drug at the onset of chest pain during continued bicycle exercise provides a suitable test model to determine the time of onset of action in exertional angina pectoris.