A patho-histological observation has been conducted on the tooth and the surrounding tissues in 63 healthy albino rats subjected to acute or chronic poisoning through administration of para-aminodiethylaniline sulphate. The drug was injected on the back skin of animals in amount ranging from 0.05 to 0.00625 gram per kilogram of body weight once in a day for 300 days. The results were summarized as follows. 1) Although p-amino-n-diethylaniline is a derivative of p-phenylendiamine, it failed to produce the marked edema in the face and neck of the experimental animals which has been claimed to be characteristic of poisoning by the latter agent. 2) By administrating 0.05 gram, approximately fatal dosage of the drug, the body weight of the animals decreased and sometimes increased, but this increase was preceded by a period of temporary decrease. 3) The drug proved to cause chiefly cardiac disturbance in the animals. The excretion of the drug apparently took place through kidney, salivary gland and lung where marked degenerative changes, particularly those of the kidney, were observed. 4) Changes in the tooth and the surrounding tissues. a) There were observed in the space between layers of ameloblasts and undecalcified enamel substanceat the site of enamel formation, drop-like, sometimes granular and rarely sheet-like foreign matters deposited, whereas no appreciable change was noticed in the enamel substance. b) Pathologic change of ameloblasts in the teeth of animals poisoned to death on the first day of administration was scarsely noted. However, by approximately 10 days of poisoning, there occur a variety of changes such as irregurarity of arrangement, curving of protoplasm and decreased staining reaction of cell nuclei in the section of ameloblast layer extending from its formative site through part of decalcified enamel substance. At the same region of tooth, the outer ameloblasts alsos suffered atrophy, irregular arrangement of the cells and vacuolar degeneration of pectinate cells. Besides this, at the site of tooth which comes on the same level with top margin of alveolar process, and where both outer and inner laysrs of ameloblast meet, vacuolar degeneration and cyst formation were obser rved in some cases, and in other hypertrophy of outer ameloblast cells were frequently observed. c) In the dental pulp, congestion, sometimes small haemorrhage in the pulp of the molar teeth, were observed, and in severer congestion, the pulp became edematous, accompanied by atrophy and decreased unmber of the ground cells. The odontoblasts showed irregular arrangment of cell, slight vacuolar degeneration and decreased staining reaction, and in those of the molar teeth reticular degeneration was sometimes observed. In the dentin slight sphenoidal disturbance in calcification was observed. d) In the incisal teeth, congestion and edematous rarefaction of the connective tissue sur rounding blood vessels were noticed in the tooth sac and tooth membrane. Greater amou unt of blood was contained in the bone marrow of the alveolus. e) No marked change was observed occurring in the tooth cementum and the gingiva. f) The foregoing findings in the tooth and the surrounding tissues would seem to imply not so much specific changes due to the poisoning by this drug as one of the secondary effects upon these tissues resulting chiefly from the disturbance of blood circulation and the aubsequent damage of the main internal organs which were more or less manifested in the animals poisoned.