Light and electron microscopic morphometry was carried out on an autopsy patient with olivary enlargement and secondary degeneration of the central tegmental tract due to a pontine tegmental lesion. The authors found active participation of neuropile astrocytic processes in the early stage of olivary enlargement before the appearance of gemistocytic astrocytes. The presence of a new stage between the stages of olivary hypertrophy and culminant olivary enlargement was clarified for the first time through electron microscopic and morphometric analysis.