Interruption of normal bursting activity by application of a voltage clamp reveals that action potentials in Aplysia neurone R15 are followed by two slow currents that long outlast the currents produced during the action potentials. Similar currents are seen following simulation of an action potential with a brief depolarizing pulse delivered under continuous voltage clamp. One of these currents, herein called ID, is an inward, or depolarizing current 0.5-5 nA in amplitude that reaches a peak 300-500 ms after the action potential. It produces the depolarizing after-potential that follows action potentials in this cell and is responsible also for the grouping together of action potentials into bursts. The second current, herein called IH, is an outward, or hyperpolarizing current 0.1-2 nA in amplitude that reaches a peak in 2-10 s and is still present for many tens of seconds following the action potential. IH mediates the interburst hyperpolarization. Both currents summate temporally during the burst. Despite changes in the amplitude and duration of action potentials during the burst, each action potential adds nearly constant increments to the summated amplitudes of ID and IH. The summated amplitude of ID grows during the first few action potentials and gives rise to the increased rate of depolarization and the increased firing rate seen during the first half of the burst. Due to its slower kinetics, IH summates throughout the burst until its summated amplitude is large enough to cause the cell to hyperpolarize, thereby bringing the burst to an end. When the normal burst is interrupted by application of the voltage clamp, the ID and IH current peaks are followed by a current which approaches a more negative steady-state level with a time course that consists of at least two phases. The first phase is exponential with a time constant of 15-30 s. Under continuous voltage clamp, the current following a train of depolarizing pulses returns to the holding current with a similar time course. These observations, together with time constants for IH that are longer than the interburst interval, suggest that IH is always partially activated during normal bursting. A computer simulation demonstrates that opposing inward and outward currents with different kinetics, i.e. ID and IH, are sufficient to give rise to bursting activity, in the absence of non-linear voltage-dependent conductances. Such voltage-dependent conductances, which are present in the normal cell, contribute to but are not necessary for bursting activity.