A self-adjustable holder for automatically positioning electroencephalographic electrodes on the head of an animal in accordance with the International 10/20 System. This holder comprises a cap which is expandable and elastic so as to self-adjust to a snug fit when in place on the head, and a plurality of electrode positioning elements located on and attached to the cap at various points thereon. Each electrode positioning element is designed to hold an associated electrode in contact with the head at a precise position, with respect to at least one reference point on the head, in accordance with the International 10/20 System.