1. Potassium currents were studied under voltage-clamp conditions in nerve cell bodies of the nudibranch Tritonia diomedia. 2. Potassium currents could be separated into three distinct components on the basis of their sensitivity to 4-aminopyridine (4-AP), tetraethyl-ammonium (TEA) and to Co2+ and Mn2+ ions. 3. A transient potassium current, similar to the fast outward current described by Connor & Stevens (1971b) and Neher (1971), was blocked by externally applied 4-AP but was much less sensitive to TEA or to Co2+ or Mn2+. A single 4-AP ion binds each receptor with an apparent dissociation constant of 1-5 X 10(-3) M. 4-AP decreases the rates of activation and inactivation and reduces the maximum conductance of transient current channels. 4. Delayed outward current was not effected by 4-AP at concentrations which blocked the transient current, but it could be divided into two components by external application of TEA and Co2+ or Mn2+. 5. A voltage-dependent component of delayed current, termed K-current, was blocked by TEA. Each K-current receptor binds a single TEA ion with an apparent dissociation constant of 8 X 10(-3) M. Co2+ and Mn2+ have little or no effect on K-current. 6. A second component of delayed outward current, termed C-current, depends on Ca2+ entry for its activation. It is similar to the Ca2+ dependent potassium current reported by Meech & Stranden (1975) in Helix cells. C-current is essentially blocked by 30 mM external Co2+ or Mn2+. It is little affected by TEA, however, being reduced by about 20% at a TEA concentration of 100 mM. 7. It is concluded that three sets of potassium selective channels contribute to the outward current and that these channels can be separated pharmacologically.