Block by paxilline exhibits an inverse dependence on Po, but no dependence on Ca²⁺ per se. G-V curves were generated under three equilibration conditions (A: 10 µM Ca²⁺, 0 mV; B: 300 µM Ca²⁺, 0 mV; C: 300 µM, −70 mV), with either control saline or either of two paxilline concentrations, 10 or 100 nM. For all patches included in this dataset, each patch was examined under all conditions. Paxilline was applied for 4 min before generation of G-V curves. Among different patches, the sequence of holding conditions was varied. G-V curves were normalized to the maximal conductance observed in the absence of paxilline under each holding condition. Curves to control points are fitted single Boltzmann functions, whereas curves in the presence of paxilline have no significance. (D) The fractional block for voltages from 40 to 120 mV was determined for each paxilline application under the three conditions and plotted in relation to the Po at the holding condition defined by the fitted Boltzmann. For 10 µM Ca²⁺ and 0 mV, the effective Po was 0.02; for 300 µM and −70 mV, the Po was 0.07; and for 300 µM Ca²⁺ and 0 mV, the Po at the holding condition was 0.48.