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1. The noise included in the Na current and the anomalous rectifier current was measured in intracellularly perfused eggs of Halocynthia roretzi Drashe under voltage clamp conditions. 2. The time-dependent component of the point-by-point ratio between the ensemble mean and the variance of successive current traces was the mirror image of the time-dependent component of the mean current itself. This can be explained by assuming an open or closed conductance state for each ionic channel, and permits estimation of unitary currents and channel number. 3. The unitary Na currents showed a potential dependence similar to that of the instantaneous currents. The unitary conductances (gamma) at zero membrane potential were 7.4 and 3.3 pmho in sea water with 400 and 100 mM-Na, respectively, and 400 mM-intracellular K. With 400 mM-Na intracellular perfusate and 400 mM-Na sea water, gamma was 15.7 pmho. In spite of the large differences in the unitary conductances, the single channel permeability for Na ions remained constant (1.15 x 10(-14) cm3 sec-1). The density of Na channels was 0.60/micrometer2. 4. The unitary currents calculated for the anomalous rectifier showed a potential dependence similar to that of the zero-time extrapolated estimates of the K inward currents. The anomalous rectifier unitary conductance was 5.5 and 6.9 pmho for sea water with 50 and 100 mM-K, respectively. The density of these channels was 0-035/micrometer2.

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