Previous studies showed that amiodarone causes state-dependent inhibition of Na(+) channels thereby mediating an atrial-selective drug effect. The aim of the present study was to investigate the impact of the new antiarrhythmic compound dronedarone on Na(+), Ca(2+) and hyperpolarization-activated cyclic nucleotide-gated ion channels. Monophasic action potentials (MAP) and effective refractory period (ERP) were studied in arterially perfused left atria and ventricular wedge preparations of the pig. Fast Na(+) and Ca(2+) currents in isolated guinea pig ventricular myocytes as well as human HCN4 channels expressed in Chinese hamster ovary (CHO) cells were investigated with the patch-clamp technique. In left atrial epicardial tissue, dronedarone (3 μM) had no effect on the MAP duration, but the drug caused a significant prolongation of the ERP from 145 ± 9 to 184 ± 17 ms (n = 6; p < 0.05). In guinea pig ventricular myocytes, dronedarone exhibited a state-dependent inhibition of the fast Na(+) channel current with an IC(50) of 0.7 ± 0.1 μM, when the holding potential (V (hold)) was -80 mV. The maximal block at the highest concentration used was 77 ± 8%. In contrast, when V (hold) was -100 mV, inhibition with 10 μM dronedarone was only 9 ± 3% (n = 7). Dronedarone blocked Ca(2+) currents elicited by rectangular pulses at V (hold) = -40 mV with an IC(50) value of 0.4 ± 0.1 μM (maximal block by 10 μM dronedarone, 80 ± 6%), whereas at V (hold) = -80 mV, 10 μM dronedarone blocked only 20 ± 6% (n = 4) of the current. Applying an action potential clamp (V (hold) = -80 mV) yielded an IC(50) of 0.4 ± 0.3 μM. Human HCN4 channels expressed in CHO cells were blocked by dronedarone with an IC(50) of 1.0 ± 0.1 μM. Inhibition of fast Na(+) and Ca(2+) channels by dronedarone depends on the cell's resting membrane potential (state-dependent block) favouring an atrial-selective mode of action. Besides fast Na(+) and Ca(2+) channels, dronedarone also inhibits HCN4 currents. This might contribute to the clinically observed reduction in heart rate seen in patients in sinus rhythm after dronedarone treatment.