The present study describes the identification and the ultrastructural and numerical evolution of Purkinje cell axonal swellings induced by phenytoin. Thirty male C57Bl/6J mice received phenytoin orally in doses up to 100 mg/kg daily and were killed after 3, 6, 10, 14, and 48 days of treatment. Light and electron microscopic investigations as well as morphometric analysis of cut surface area and numerical density of axonal swellings were performed. The swellings appeared as early as 6 days after initiation of treatment and gradually increased in size and frequency. Use of an anti-lymphocyte monoclonal antibody (CD 3), specifically cross-reacting with Purkinje cells, identified the swellings as dystrophic Purkinje cell axons. On grounds of their ultrastructural appearance they were classified into three distinct types occurring at different time intervals after phenytoin exposure. At 6 days, most axonal swellings contained loosely aggregated membranous vesicles and tubules in a finely granulated matrix (type 1). At 14 days, larger axonal swellings appeared characterized by the presence of three-dimensional networks of branched and anastomosing membranous tubules (type 2). At 48 days, even larger axons contained bodies of highly condensed membranous material of sometimes paracrystalline appearance (type 3). It is suggested that phenytoin-induced axonal pathology of Purkinje cells is a dynamic process characterized by the progressive accumulation of proliferating membranous material arranged in an increasingly complex fashion.