Planar lipid membranes in the presence of the ion carriers valinomycin or nonactin were irradiated with 14 MeV electrons from a linear accelerator. A large increase of the membrane conductance by up to more than two orders of magnitude was found. The effect is virtually abolished either at high pH, or in the absence of oxygen, or in the presence of the radical scavenger ethanol. A further prerequisite for the effect is the presence of unsaturated fatty acid residues. A kinetic analysis of the carrier transport model based on current-voltage curves and on voltage-jump relaxation experiments was performed as a function of radiation dose. Only the translocation rate constant, kMS, of the charged carrier-ion complex was found to be influenced by irradiation. The effect is interpreted as an increase of the polarity (dielectric constant) of the membrane interior induced by the presence of polar products of lipid peroxidation. A combined action of OH- and HO2-radicals seems to be responsible for the phenomena. At large radiation doses (greater than or equal to 10(3) Gy) a reduction of the membrane conductance was observed. This is interpreted as an increased microviscosity, possibly caused by cross-linking of fatty acid residues. Ion carriers represent sensitive probes of radiation induced membrane damage.