Sodium selenite administered to rats in drinking water, at a dose of 0.40 ppm for 12 days, caused proliferation of the rough and smooth endoplasmatic reticulum and their necrosis. These changes were most intensive 24 hours p.i. and persisted until 72 hours p.i. They were accompanied by mitochondria intumeseence, their partial degradation and glycogen atrophy, most noticeable 72 hours p.i. Diazinon adrninisteredper as at a dose of 40 mg/kg b.m. twice at 6-day intervals caused necrosis of the rough and smooth endoplasmatic reticulum. The changes set in at a slower pace than following selenium administration and none were observed 14 days p.i.Treatment with sodium selenite and subsequent exposure to diazinon induced ultrastructural changes in the rat hepatocytes, which were more intensive and accompanied by an intumeseenee of Golgi complex and cell vacuolisation. In these cases the submicroscopic changes were most noticeable 1 and 3 days p.i. Necrotic hepatocytes were observed 14 days p.i
