The Ca current of the egg cell membrane of a certain tunicate, Halocynthia roretzi Drasche, was studied by the voltage-clamp technique. 2. The Ca current in the standard artificial sea water (ASW) was produced at the critical membrane potential of -10 mV after inactivating the Na current by conditioning depolarization, -30 to -15 mV. The Ca current was abolished by replacing Ca in ASW with Mg2+ or Mn2+. The Ca current was not significantly influenced by replacing Na in ASW with choline or Cs. 3. The relation of Ca current to the external Ca concentration was a monotonously increasing function, but was not linear. The current tended to saturate above 50 mM-Ca. In 100 mM-Ca ASW, the maximum peak inward current of Ca ranged from 1 to 7 X 10(-9) A. 4. The kinetics of Ca current was accurately analysed because of the small contribution of K outward current and was found to be relatively slow in comparison with the Na current. The peak time and the half-decay time of the maximum Ca current at about 25 mV were about 25 and 100 msec respectively in 100 mM-Ca ASW at 15 degrees C. 5. Addition of 20 mM-Co2+ to 100 mM-Ca ASW reduced Ca current to one fourth and 1 mM-La3+ to 100 mM-Ca ASW abolished the current. 6. Sr and Ba could substitute for Ca in Ca channels. The selectivity ratios for the 'Ca channels's were Ca (1-00):Sr(1-17):Ba(0-71) at a potential level of +40 mV. The Ca current in the egg cell membrane appeared to be essentially the same as the Ca current in the common excitable membranes, such as the crustacean muscle fibre. 7. The polyvalent cations including Ca ion and monovalent H+ ion showed the stabilizing effect upon both Na and Ca currents, by shifting V-I relations along the voltage axis. From the prediction of a theory of the diffuse double layer, the shift in the V-I relation induced by those cations should be directly related to their binding powers to the membrane. Thus, the sequence of the binding powers was inferred as H+ greater than La3+ greater than Co2+ greater than Mn2+ greater than Ca2+ greater than Sr2+ larger than or equal to Ba2+ greater than Mg2+. 9. In Na-free ASW, such as isotonic Ca ASW, Ca current was composed of two components. The one component was the Ca current described in 1 to 6. The other was also dependent upon the external Ca concentration, but showed the more negative critical membrane potential and the faster kinetics. It was concluded that this component should be the Ca current through Na channels. 10. The selectivity among Ca, Sr and Ba for 'Ca' current through 'Na channels' was significantly different from that of 'Ca' current through 'Ca channels', being Ca greater than Sr larger than or equal to Ba = 0.