The morphologic and functional effects of free radicals on renal cells in vitro were investigated, as well as the possibility of avoiding them by pretreatment with scavenger enzymes or a xanthine oxidase inhibitor. Cultured human kidney cells, incubated together with a free radical-generating system, with and without protective agents, were examined by light and scanning electron microscopy. The vimentin filament structure of the incubated cells was visualized by immunofluorescence. The membrane function was studied in human kidney cells by using a dye exclusion test and in rabbit kidney slices by determination of the sodium-potassium pump activity. Exposure of the cells to free radicals caused rapid development of severe morphologic lesions, including extensive cytoskeletal disorganization. After pretreatment, only a few cells had similar, although less severe, lesions. The results of the dye exclusion test and indirect evaluation of the sodium-potassium pump activity did not indicate any major damage to the cell membranes after exposure to free radicals.