1. The properties of the fast-inactivating or transient K current and the slowly inactivating or delayed K current of the membrane of immature eggs of the clonial marine coelenterate, Renilla Koellikeri, were studied by using voltage clamp and intracellular dialysis techniques. 2. The transient current is activated when the membrane potential becomes more positive than -25 approximately -20 mV (resting potential, -72 +/- 5 mV) whereas the activation potential of the delayed current is -10 approximately OmV. These potentials are independent of either [k+]o or [K+]i. 3. The inactivation of the transient current is rapid and is almost complete for membrane potentials more negative than the activation potential while it is slow for the delayed current and incomplete within a few seconds. 4. Both currents shows similar reversal potentials which are predominantly determined by the K concentration gradient across the membrane. 5. The sensitivities of the conductance upon the internal K concentration differ between the two currents, suggesting that the interaction between the site and ions in the membrane channels differ between them. 6. Neither current is a Ca-activated K current. 7. 4-AP suppresses the transient current at concentrations substantially smaller than those that suppress the delayed current while TEA shows no effect on either current. 8. Intracellular application of pronase or tannic acid at relatively high concentrations does not alter the inactivation of either current. 9. The membrane includes a voltage-dependent Ca permeability which results in action potentials under current-clamp conditions.