Hydropic alterations of the fine structure of the choroid plexus epithelium following administration of three different tertiary amines to rats were compared with alterations produced by cyclophosphamide and with normal choroid plexus. Single doses of 2, 7-bis(2-(diethylamino)-ethoxy)- 4− methyl-1,8-naphtyridine HCI or 2,6,-di-(omega-dimethyl aminoethoxy)-pyridine produced dramatic accumulations of membrane-limited vacuoles. Multiple doses of 2,7-bis-(diethylaminoethoxy) fluoren-9-one HC1 (tilorone) produced accumulations of dense cytoplasmic bodies in choroid epithelia cells in addition to the hydropic vacuoles. Differential responses to these agents suggested functional specialization or temporal variation in function among adjacent choroidal cells. Despite the occurrence of pronounced plasma exudation in the plexitis following cyclophosphamide treatment, there was no hydropic vacuolization.