Conduction of the sino-atrial impulse from the high right atrium to the ventricles was studied by intracardiac electrography in 21 unoperated patients, age 3 months to 11 years, with endocardial cushion defects (ECD). The high right atrium-to-low right atrium conduction time was prolonged in 15 of 18 subjects (mean 57 +/- 20 msec). The low right atrium-to-His bundle conduction time (LRA-H) was normal in 16 of 17 subjects (mean 82 +/- 30 msec), prolonged in one. The His-to-ventricle conduction time (H-V) was normal in 16 of 17 subjects (mean 37 +/- 8 msec), equivocally short in one. Nine patients with ECD, age 3 to 21 years, were studied postoperatively. One had an acquired complete atrioventricular block in the His bundle. Two had prolonged LRA-H and two prolonged LRA-H and two prolonged H-V. The surface ECG failed to identify accurately either prolonged atrioventricular conduction or the site of prolongation.