Potassium current through the inward rectifier of Neanthes arenaceodentata eggs was studied using a voltage-clamp technique. The instantaneous conductance, steady-state conductance and the time constant of current relaxation were analysed as functions of the membrane potential and the external potassium concentration ([K]o). Both the instantaneous and the steady-state conductances increased sigmoidally with hyperpolarization, reaching saturation values at potentials 40 mV more negative than the potassium equilibrium potential (EK). The time-dependent change in conductance followed first-order kinetics throughout an 80 mV potential range centred at EK. The conductances increased with time during hyperpolarizations and decreased with time during depolarizations. The time constant decreased sigmoidally about EK as the membrane potential (Vm) was made more positive. The results are interpreted in terms of two models. The first model assumes single-channel rectification. Forcing the data to conform with this assumption necessitates that the single-channel conductance, the open-channel probability, and the time constant of current relaxation all be nearly identical functions of Vm--EK. This coincidence is considered implausible. The second model ascribes the apparent instantaneous rectification to a fast kinetic process. This model correctly predicts the steady-state conductance, the time-constant of current relaxation, and the unexpected proportionality between the ratio of the steady-state and 'instantaneous' conductances and the time constant of current relaxation. This agreement between data and model is obtained even though neither of the rate constants of the slow gating process has the flexibility of voltage or potassium dependence. The results are considered to imply the existence of a fast gating mechanism in the kinetics of inward rectification.