1. ZENECA ZD7288 (4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino) pyridinium chloride) is a sinoatrial node (SAN) modulating agent which produces a selective slowing of the heart rate. Its effects have been studied in single, freshly dissociated guinea-pig SAN cells, by standard patch clamp procedures. 2. Whole-cell inward currents were evoked by hyperpolarizing voltage clamp steps from a holding potential of -40 mV. ZD7288 inhibited the hyperpolarization activated cationic current (If) in a concentration-dependent manner. The 'selective bradycardic agents' alinidine and UL-FS 49 (zatebradine) both also inhibited If. 3. The activation of If was investigated by measuring tail current amplitudes at +20 mV after hyperpolarizing steps to different potentials to activate the current. The reduction in If resulted from both a shift in the If current activation curve in the negative direction on the voltage axis, and also a reduction in the activation curve amplitude. 4. ZD7288 did not affect the ion selectivity of the If channel, since the tail current reversal potential was unchanged in the presence of the drug. 5. With ZD7288 the inhibition of If was not use-dependent, whereas UL-FS 49 displayed use-dependence in the block of the If current. 6. Whereas ZD7288 had no significant effect on the delayed rectifier current (Ik) in these cells, both alinidine and UL-FS 49 significantly reduced Ik at the same concentrations which reduced If. 7. The data show that ZD7288 reduces If by affecting the activation characteristics of the If current; this inhibition may account for this agent's selective bradycardiac properties.