The study was aimed to investigate the electrophysiological properties of long QT syndrome associated with permanent bradycardia. The investigations were performed in 26 patients suffering from long QT duration (QTC-frequency adapted QT-:484 +/- 34 ms) with permanent, marked bradycardia (heart rate: 42 +/- 7 min-1). Adams Stokes syncopal attack appeared in 12 patients, while in 14 cases ventricular tachycardia attack with syncope could be observed (study group). As control served the data of 30 patients suffering from long lasting marked bradycardia (heart rate: 44 +/- 7 min-1) with normal QT (QTC:420 +/- 28 ms). Each patient was candidate for pacemaker implantation. The following questions were studied: 1. The effect of heart rate on QT duration. The experiments were performed by electrical ventricular stimulation. 2. The effect of sympathetic and parasympathetic-pharmacologic-blockade on QT time. The study was performed under electrical ventricular stimulation by administration of propranolol and atropine. 3. The dispersion of QT time was studied by using electrical heart stimulation and 12 lead ECG recording. Electrophysiological investigations were performed in 14 patients with long QT and permanent bradycardia. On augmentation of the cycle length (bradycardia) the increase in the QT duration was more-out of all proportion-expressed in long QT. On pharmacologic sympathetic blockade in long QT syndrome the QT duration significantly diminished. The QT dispersion was more expressed in patients with prolonged QT interval and on bradycardia the QT dispersion further increased significantly. The irritability of the ventricle was markedly augmented in patients with long QT and bradycardia. Appearance of polymorphous ventricular tachycardia could frequently be observed and could be regularly induced by early ventricular extrastimuli and bradycardia.(ABSTRACT TRUNCATED AT 250 WORDS)