The pharmacological test with the use of high-resolution rhythmocardiography for the analysis of cardiac rhythm variability associated with peripheral vegetative regulation of the synoatrial node was applied to study effects of a single dose of organic nitrates (nitroglycerin and mononitrate isosorbide) in 41 patients with unstable angina. Rhythmocardiograms were obtained before and after a morning sublingual dose of nitrates preceding intake of basal drugs. The results suggest negative action of nitrates on peripheral vegetative regulation of pacemaker activity of the synoatrial node.