A method for recording electrical activity of the sinoatrial (S-A) node and automatic atrial foci in human subjects is described. To record S-A nodal electrograms, an electrode catheter was inserted percutaneously into the femoral vein and advanced under fluoroscopic control to the superior vena caval-right atrial junction. The distal terminal of the catheter was placed in the area of the S-A node and the proximal terminal on the free right atrial wall or in the right atrial lumen. Polarity was reversed from the conventional electrocardiographic recording; high amplification (about 100 microV/cm) and selective filters (0.1 to 20 hertz) were used. S-A nodal electrograms recorded with this method in human subjects were similar to electrograms obtained previously from the dog and rabbit and revealed negatively directed diastolic and upstroke slopes preceding the P wave of the electrocardiogram. Sinoatrial conduction time measured from the S-A nodal electrograms in 15 cases was 34.9 +/- 2.1 ms(mean +/- standard error of the mean) for a sinus cycle length of 736.4 +/- 38.6 ms. The coronary sinus electrograms in a patient with coronary sinus rhythm were recorded by the same technique except that the distal terminal of the catheter was placed at the coronary sinus ostium. A negatively directed diastolic slope preceding the P wave was consistently recorded. This method for recording electrograms of the S-A node and ectopic automatic atrial foci should prove useful in (1) assessment of both normal and abnormal S-A nodal function, (2) direct determination of conduction time from the S-A nodal pacemaker to the atrium, and (3) localization of automatic atrial foci.