The high-pass filtering of small signals by the rod photoreceptor network was studied by intracellular recording in the isolated, perfused retina of the toad, Bufo marinus. Data were analyzed and interpreted in terms of the network analysis described in the preceding paper. External concentrations of Cs+ as high as 10 mM, which blocked the relaxation from peak to plateau of the rod's response to bright light, did not affect the filtering of small signals. The effects of reducing [Na+]o were not consistent with a direct action upon the mechanism underlying this filtering property. By contrast, raising external [K+] from 2.6 to 10 mM, which caused a fourfold reduction in EK, abolished the high-pass filtering of small signals. Analysis of the effects of external [K+] changes indicates that the underlying mechanism involves a K+ conductance that decreases with a delay when the rod is hyperpolarized. This conductance is not blocked by externally applied tetraethylammonium. Other experiments did not rule out the possibility that it might be activated by Ca++.