1. A voltage-dependent proton current, IH, was studied in cultured myotubes obtained from biopsies of human muscle, using whole-cell recording with the patch-clamp technique. 2. With a pHo of 8.0 and a calculated pHi of 6.3, IH was activated at voltages more depolarized than -50 mV and its conductance reached its maximum value at voltages more depolarized than +10 mV. 3. Studies of the reversal potential of IH during substitution of K+, Na+, Ca2+, Cl-, Cs+ and H+ in the extracellular solution indicated that protons were the major charge carriers of IH. 4. IH was also activated during a voltage step to +22 mV with a pHo of 7.3 and a calculated pHi of 7.3. 5. Acidification of the extracellular solution led to a shift towards depolarized voltages of the conductance-voltage relationship. 6. Stationary noise analysis of IH suggested that the elementary event underlying IH was very small with a conductance of less than 0.09 pS. 7. Extracellular application of various divalent cations blocked IH. The block by divalent cations was voltage dependent, being more efficient at hyperpolarized than at depolarized voltages. For Cd2+, the Michaelis-Menten constant (Km) for the block was 0.6 microM at -28 mV and 10.4 microM at +12 mV. 8. Ca2+ was a less efficient blocker than Cd2+ but could block IH at physiological concentrations (the Km values for the block were 0.9 mM at -38 mV and 7.3 mM at -8 mV). 9. The voltage-dependent properties of IH and its ability to be affected by pH and Ca2+ suggest that IH might be used by skeletal muscle cells to extrude protons during action potentials. 10. A model of IH activation suggests that under extreme conditions, the conductance of IH can reach 40% of its maximum value after less than ten action potentials.