The mechanism of increase in the inner membrane permeability induced by Ca2+ plus Pi, diamide and hydroperoxides has been analyzed. (1) The permeability increase is antagonized by oligomycin and favoured by atractyloside. The promoting effect of atractyloside is strongly reduced if the mitochondria are simultaneously treated with oligomycin. (2) Addition of the free-radical scavenger, butylhydroxytoluene, results in a complete protection of the membrane with respect to the permeability increase. (3) Although membrane damage and depression of the GSH concentration are often associated, there is no direct correlation between extent of membrane damage and concentration of reduced glutathione. Abolition of the permeability increase by butylhydroxytoluene or by oligomycin is not accompanied by maintenance of a high GSH concentration in the presence of diamide or hydroperoxides. The membrane damage induced by Ca2+ plus Pi is not accompanied by a depression of the GSH concentration. (4) It is proposed that a variety of processes causing an increased permeability of the inner mitochondrial membrane merge into some ultimate common steps involving the action of oxygen radicals.