Thyroid hormones (TH) are powerful modulators of heart function, but their arrhythmogenic effects are less elucidated. We have examined both acute and long-term action of TH on the heart susceptibility to the ventricular fibrillation (VF) and on the heart ability to terminate VF and restore a sinus rhythm. Triiodothyronine (T3) was applied in the range of 10(-9)-10(-6) mol/l in acute experiments using isolated perfused aged (14-month-old) guinea pig hearts. L-thyroxine (T4) was applied in the dose of 50 microg/100g/day to young (3-month-old) and aged (20-month-old) rats for 2 weeks. The T4 treatment resulted in an increased susceptibility of young, but not adult rat hearts to a hypokalemia-induced VF and facilitated a spontaneous sinus rhythm (SSR) restoration in the latter group. The acute T3 administration in the range of 10(-9)-10(-7) mol/l significantly decreased the susceptibility of an isolated heart to an electrically induced VF and also facilitated the sinus rhythm restoration. The SSR restoration was, however, not affected by 10(-6) mol/l concentration of T3, which also led to an increased VF susceptibility. Results indicate that TH can affect the susceptibility of the heart to VF and its ability to restore the sinus rhythm via acute (non-genomic) and long-term (genomic) actions. Furthermore, an anti- and pro-arrhythmic potential of TH appears to be age- and dose-dependent.