Membrane currents and membrane potentials of the fresh-water ciliate Stylonychia mytilus were investigated by voltage-clamp and constant-current injection techniques. 2. The Ca-dependent action potential of Stylonychia in a solution containing 0.1 mM-CaCl2 was prolonged by the addition of Mg or Na ions. 3. In a nominally Ca-free solution, containing 2 mM-MgCl2, the cells generated repetitive, spontaneous action potentials of relatively small amplitude (17 mV). The addition of 0.5 microgram concanavalin A/ml completely inhibited these action potentials in 2 mM-Mg. 4. In voltage-clamp experiments in standard solution, the inward current-voltage relationship has two maxima, confirming the existence of two different voltage-dependent Ca currents in Stylonychia: inward current I and II. In a nominally Ca-free, Mg-containing solution, the remaining inward current was inhibited by concanavalin A, a specific inhibitor of inward current I. No residual second inward current (current II) was detected in a solution containing Mg and concanavalin A. 5. Experiments, with altered ratio of Ca and Mg ions and constant concentration of divalent cations (mole-fraction experiments), showed that Mg and Ca do not inhibit each other's passage through channel I. Calculations assuming a Ca-channel model with one cation-binding site per ion channel I showed good correlation with the experimental data. 6. A similar inward current was seen after replacement of Mg by Na in nominally Ca-free solution.
