Atrioventricular conduction characteristics were examined by atrial extrastimulus technique in 45 children aged 50 days to 14 years following diagnostic cardiac catheterization. None of the patients had a history of supraventricular tachycardia and none had evidence of pre-excitation on routine electrocardiogram. By plotting the A2H2 and H1H2 intervals against corresponding S1S2 intervals 2 types of conduction curve were obtained. Eight of the 45 children (17.7%) showed discontinuous conduction curves, characteristic of dual atrioventricular nodal pathways, whereas 37 patients had continuous curves. Of the 8 patients with discontinuous curves 3 showed unsustained echo beats following extrastimuli, after the effective refractory period of the fast pathway was reached. This study showed that, as in adults, the atrioventricular node was frequently functionally dissociated in children.