Intracellular acidosis, at constant extracellular pH, hyperpolarizes the resting potential and reduces the diastolic depolarization rate of cardiac Purkinje fibres. With alkaline pHi, the fibre depolarizes and spontaneous firing is observed. Intracellular pH transients induced either by superfusion with Tyrode buffered with 5% CO2/23 mM HCO3- or 16% CO2/61 mM HCO3-, or with solutions containing weak undissociated acids, transiently shifted the half-maximum activation potential E0.5 of the pace-maker current. Similar transients were observed when NH4Cl was added and subsequently withdrawn from the solution. Simultaneous pHi measurements demonstrate a close relation between the time course of the pHi and E0.5 variations. Acid pHi shifts E0.5 to more negative and alkaline pHi's to less negative potentials. These pace-maker current activation voltage shifts are interpreted as the direct consequence of fixed charges titration at the inside of the sarcolemma. Other effects, like the slowing-down and reduction of the pace-maker current by acid pHi, presumably result from other interactions of protons with the pace-maker channel.