His bundle electrograms were performed on 10 patients with organic heart disease. Six patients had had a recent myocardial infarction. Recordings were made at various rates utilizing right atrial pacing. Nitroglycerin, 1/150 gr, was administered to all 10 subjects, and the P-A, A-H, H-Q and H-S intervals were determined before, and immediately after the disappearance of the sublingually administered nitroglycerin. A significant decrease in the A-H interval occurred with negligible effects on the P-A, H-Q and H-S intervals. At the atrial pacing rate of 100/min, the average A-H interval fell from the control value of 152 msec to 129 msec after the administration of nitroglycerin (p less than 0.02); at the pacing rate of 130/min, the A-H interval decreased from 194 to 133 msec (p less 0.05). Second degree heart block occurred at higher pacing rates in six patients after nitroglycerin administration as compared to the control value. The average postsuppression sinoatrial recovery time control value of 1,083 msec decreased to 906 msec after nitroglycerin administration (p less than 0.01). These findings demonstrate that nitroglycerin can improve conduction through the A-V node.