Single sinus venosus cells from frog, Rana esculenta, were isolated using an enzymic dispersion procedure, involving applications of collagenase and protease. About 40% -60% of the cells showed spontaneous contractions. Isolated cells were studied in the wholecell configuration. Regenerative action potentials were tetrodotoxin-insensitive and similar to those recorded in multicellular preparations. Hyperpolarizing pulses in the voltage range negative to -50 mV induced the activation of a time-dependent inward current, which was blocked by 4 mM caesium but less affected by barium ions. A lower concentration of caesium (1 mM) exerted a voltagedependent reduction of the current and decreased the spontaneous pacing rate. The activation range of the hyperpolarization-activated current approximately extended from -50 mV to -110 mV, but varied from cell to cell. A high variability was observed in the behaviour of the activation kinetics. The current had a reversal potential near -20 mV" that was shifted positively by increasing the external potassium concentration (from 3 mM to 30 raM) and negatively by reducing the external sodium concentration (from 115 mM to 30 raM). The hyperpolarization-activated inward current of the frog sinus venosus cell appears to be carried by both sodium and potassium ions. It shows electrophysiological properties similar to those of the If current of the mammalian heart. The role of the current in the spontaneous activity is discussed.