Using the method of bilayer membranes at the tip of a patch pipette, the properties of the ionic channels produced by the ionophore toxin pardaxin were investigated. At low toxin concentrations, voltage-dependent, single-channel events were measured. The current-voltage curves were non-linear when determined in Tris-Cl solution, but were linear in K(+)-HEPES solution. Using asymmetric ion solutions, the ionic selectivity of pardaxin channels was estimated from the reversal potentials obtained. The sequence of the relative permeabilities for monovalent cations was Tl+ > Rb+ > Cs+ > K+,NH4+ > methylamine+ > Li+ > dimethylamine+ > Na+. Except for Li+, the selectivity sequence fitted the cations relative hydrated size. For bivalent ions the permeability of Ba2+, Sr2+, and Mn2+ relative to Mg2+ changed according to the relative hydrated size. For anions the selectivity sequence was I- > NO3- > Br- > Cl- > ClO4- > SCN- > BF- > HCOO- > F- > CH3COO-. The selectivity sequence for the small anions (I-, NO3-, Br-, Cl-) was different from their hydrated size. Pardaxin channel showed a modest ion selectivity between small anions and cations (PK:PCl:PNa = 1.28:1.00:0.56). Pardaxin is proposed as a biophysical model to study ionic channel selectivity.