Method and apparatus for replacing flammable, insulating oil in an oil-filled electric cable by non-flammable, insulating oil in which the flammable oil is withdrawn at least from the cable oil duct under vacuum and with heating of the cable to its maximum operating temperature and thereafter, the flammable oil which was withdrawn is replaced by non-flammable oil. During operation of the cable, oil which flows out of the cable during heating of the cable is sent to a first reservoir, through a check valve and during subsequent cooling, such oil is replaced by non-flammable oil from a second reservoir containing non-flammable oil and connected to the cable duct by a check valve. Also, an oil-filled cable having its insulation impregnated with flammable, insulating oil or a mixture of such oil with non-flammable, insulating oil and having an oil duct filled with a non-flammable, insulating oil which is different from the flammable oil.