1. The effect of the bradycardic agent S 16257 on the main ionic mechanisms of diastolic depolarization in sinoatrial node cells isolated from rabbit heart, was investigated by the patch-clamp technique in whole-cell and macro-patch recordings. 2. In whole-cell conditions, S 16257 induced a marked exponential use-dependent blockade of the hyperpolarization-activated I(f) current, without shift of the voltage range of its activation curve. The rate of block increased with the drug concentration. The IC50 for the block of I(f) was 2.8 x 10(-6) M. 3. A similar use-dependent decline of I(f) was obtained with 3 microM S 16257, in cell-attached and in inside out macro-patch configurations, suggesting that the bradycardic agent interacts with I(f) channels from the inside of the cell. 4. A high concentration of S 16257 (10 microM) had no detectable effect on T-type calcium current and slightly decreased L-type calcium current (-18.12 +/- 0.66%), without significant use-dependent blockade. 5. S 16257 had no effect on the delayed outward potassium current Ik at 3 microM and slightly decreased it only at high concentrations, -16.3 +/- 1.2% at 10 microM. In contrast, zatebradine, another bradycardic agent, reduced I k by 20.3 +/- 2.5% at 3 microM. 6. In conclusion, S 16257 may lower heart rate without significant negative inotropic action. In comparison with zatebradine, S 16257 had less effect on Ik suggesting less prolongation of repolarization time.