Outward and inward currents through single small-conductance K(+) (SK) channels were measured in cell-attached patches of the apical membrane of principal cells of the rat cortical collecting duct (CCD). Currents showed mild inward rectification with high [K(+)] in the pipette (K(p)(+)), which decreased as K(p)(+) was lowered. Inward conductances had a hyperbolic dependence on K(p)(+) with half-maximal conductance at approximately 20 mM. Outward conductances, measured near the reversal potential, also increased with K(p)(+) from 15 pS (K(p)(+) = 0) to 50 pS (K(p)(+) = 134 mM). SK channel density was measured as the number of conducting channels per patch in cell-attached patches. As reported previously, channel density increased when animals were on a high-K diet for 7 days. Addition of 8-cpt-cAMP to the bath at least 5 min before making a seal increased SK channel density to an even greater extent, although this increase was not additive with the effect of a high-K diet. In contrast, increases in Na channel activity, assessed as the whole cell amiloride-sensitive current, due to K loading and 8-cpt-cAMP treatment were additive. Single-channel conductances and channel densities were used as inputs to a simple mathematical model of the CCD to predict rates of transepithelial Na(+) and K(+) transport as a function of apical Na(+) permeability and K(+) conductance, basolateral pump rates and K(+) conductance, and the paracellular conductance. With measured values for these parameters, the model predicted transport rates that were in good agreement with values measured in isolated, perfused tubules. The number and properties of SK channels account for K(+) transport by the CCD under all physiological conditions tested.