1. The kinetics of the anomalous K current produced by blocking cations Na, Cs and Sr were analysed by single channel recording in the tunicate egg cell.2. The open-close kinetics in a single channel with the presence of blocking cation were consistent with the blocking kinetics of the total anomalous K current. The open-close kinetics in a single anomalous K rectifier channel could be regarded as the first order transition between the open and closed states.3. The closing rate constants of the single channel for the first order transition increased almost linearly on a semilogarithmic scale as the membrane potential became more negative, while the opening rate constants decreased and then increased, showing the minimum at a certain potential level. The latter indicated that the channel was released from the blocking by excessive hyperpolarization.4. The opening rate constant increased in spite of the fixed concentration of the blocking cation (10 muM-Cs), when the external K concentration was raised from 200 mM to 400 mM. This result suggested that there are interactions within the channel between the blocking cation and the K ion.5. When there were two kinds of blocking cations such as Na and Cs, the kinetic properties in a single channel indicated that the two kinds of blocking cations blocked the single channel independently.6. The single channel conductance of the anomalous K rectifier in 200 mM-Na-containing 200 mM-K solutions was about 1.5 times larger than that in Na-free 200 mM-K solutions.7. The single channel conductances in 100, 200 and 400 mM-K solution with 10 muM-Cs were 5.0, 7.1 and 12.3 pS respectively, being roughly proportional to the square root of the K concentration.8. The density of the anomalous K rectifier channel in the tunicate egg cell was 0.039/mum(2).9. The amplitude of the single channel current increased with the rise of temperature, the Q(10) being 1.5.