1. Ionic currents in differentiated cells of mouse neuroblastoma clone N1E-115 have been studied under voltage-clamp conditions. 2. Depolarizing voltage steps from a holding potential of -85 mV to levels more positive than -40 mV produced fast transient inward currents followed by delayed outward currents. 3. The fast inward current is carried by Na+: it is blocked by tetrodotoxin and is absent in Na+-free solutions. Its kinetic behaviour resembles that of the Na+ current in squid giant axon. A mean value of 85 mmho/cm2 was found for the maximum Na+ conductance (GNa).4. The delayed outward current is carried primarily by K+: it is blocked by externally applied tetraethylammonium (TEA, 15 mM) and has a reversal potential (mean -71 mV) close to the theoretical K+ equilibrium potential. Its instantaneous I--V curve is linear. By analogy with the formulation of Hodgkin & Huxley (1952c), the outward current can be described by IK = -GKn2(V--EK) where GK = 12 mmho/mc2. 5. During prolonged depolarizations the delayed outward current declines. This decline, which occurs in two phases, represents a partial inactivation of the K+ conductance. 6. A weak inward current with slow activation and inactivation kinetics appears in Na+-free solution containing 10 mM-Ca2+. It is activated at a membrane potential of -55 mV and reaches its maximum at -20 mV with a time to peak of about 10 msec. This current is tetrodotoxin-resistant, reversibly blocked by Co2+ (5mM) and is suggested to be carried by Ca2+. 7. An increase in the external divalent cation concentration results in a parallel shift of the steady-state I--V curve along the voltage axis in positive direction. The activation of delayed outward currents is suggested not to depend on Ca2+ influx. 8. It is concluded that separate voltage-dependent Na+, K+ and Ca2+ channels exist in the differentiated neuroblastoma membrane with kinetic and pharmacological properties similar to those observed in non-mammalian preparations.