1. The properties of the 'pace-maker' current iK2 of Purkinje fibres are investigated to verify whether its behaviour during voltage-clamp pulses is consistent with the view that iK2 is a K current deactivating on hyperpolarization in the range -50 to -100 mV. 2. During voltage-clamp pulses in the range positive to the apparent reversal potential (Erev), low concentrations of Cs depress the time-dependent current change without altering the time-independent current component. The total current becomes more outward in Cs, which on the assumption that Cs only affects the iK1 and iK2 channels implies that iK2 is inward in the voltage range analysed. 3. Ba strongly reduces the time-independent component due to iK1 and therefore limits the contribution of K depletion to the total current time course during hyperpolarizations. In the presence of barium a current reversal is not obtained even with large hyperpolarizations in normal Tyrode solution. If in Ba-containing solutions the external K concentration is raised from 3 up to 48 mM, iK2 greatly increases in the inward direction during hyperpolarizations in the range -51 to -101 mV, implying that it cannot be carried by K only. 4. In the presence of Ba, measurements of the membrane conductance due to iK2 indicate a channel-opening process during hyperpolarizations and a channel-closing process during depolarizations, in the K-concentration range analysed. 5. It is concluded that iK2 is not, as had been previously thought, a pure K current, but is rather an inward current activated during hyperpolarizations negative to about -50 mV. This provides further evidence for a possible identity between iK2 in Purkinje fibres and the current if in the SA node.