The fine structural changes and the reactivity for acid phosphatase (AcPase) and thiamine pyrophosphatase (TPPase) were studied in thin sections from rat pancreatic acinar cells exposed to dl-ethionine for 2-10 days. The cells from ad libitum and pair-fed controls exhibit occasionally 0.2-0.6 microns circular profiles showing reaction for AcPase and considered as presumptive lysosomes. At days 2 and 4 of dl-ethionine treatment the acinar cells exhibit presumptive lysosomes, autophagosomes and membrane-bounded cytoplasmic areas devoid of electron density and AcPase activity, containing scattered membranous elements. These regions were named lesioned areas. On 6th, 8th and 10th days a membrane bound anomalous cytoplasmic structure that represents a dense pile of layered membrane-like material (multilayered bodies, MB) was seen. The MBs consistently show AcPase activity and in rare instances TPPase activity. Freeze fracture studies reveal that the limiting membrane of the MBs has intramembranous particles whereas the multilayered membranous contents are devoid of such particles. The structure and disposition of the lamellae of the MBs seen in the replicas are similar to those of artificially prepared phospholipidic membranes.