To compare the effects of ethmozin on sinus node (SN) function in the presence (9 patients) and absence (17 patients) of SN dysfunction, sinus cycle length (SCL), maximal corrected sinus recovery time (CSRT), paced cycle length yielding peak SN suppression, and indirect sinoatrial SA conduction time (SACT) were determined before and after intravenous administration of ethmozin in the dose 2 mg/kg. The mean +/- SD SCL were significantly shortened in patients with normal SN and were not changed in patients with SN dysfunction after ethmozin administration. The mean maximal CSRT was 252 +/- 72 before and 284 +/- 86 ms after ethmozin administration in patients with normal SN function (p less than 0.05). In patients with SN dysfunction (p less than 0.1) the mean maximal CSRT was found to be 1016 +/- 434 before and 2170 +/- 1756 ms after ethmozin administration. The mean SACT was 158 +/- 41 before and 174 +/- 51 ms after drug administration in patients with normal SN (p less than 0.05). Four out of nine patients with SN dysfunction developed second degree SA exit block after ethmozin administration, whereas SACT increased significantly in the remaining group of patients (180 +/- 35 to 210 +/- 32 ms; p less than 0.05). The PR, PA, AH, and HV intervals significantly lengthened and the valves of QRS and QT were not changed after ethmozin administration in either group. The conclusion is drawn that ethmozin should be administered cautiously to patients with SN dysfunction, particularly to patients with SA exit block, sinus pauses, or secondary pauses (in particular, with bradycardia-tachycardia syndrome).