We investigated the pathobiological course of uranyl nitrate (UN) induced polyuric acute tubular necrosis (ATN) in male Sprague Dawley rats. UN (5mg/kg 15mg/kg and 3Omg/kg) in 5% NaHCO₃ induced weight loss, polydipsia, and polyuria 24 hrs after injection when compared to the controls which were treated with 5% NaHCO₃ only. Twenty four hours following the injection of UN, serum creatinine and blood urea nitrogen levels had increased. These changes continued for at least 72 hours, although the concentration of uranium had decreased. Light microscopic studies conducted 24 hours after injection, revealed partial degeneration and necrosis of the proximal tubules and many casts m the distal convoluted tubules. These changes progressed for 72 hours. Despite this tubular damage, the glomeruli were relatively intact. 5 days after injection, the epithelial cells lining the proximal tubules displayed regenerative activities; these findings were more prominent after 10 days. Through electron microscopic examination, we observed the destruction of mitochondria in the proximal tubular cells, a possible cause of polyuria. Ten days post injection regenerative activities in the proximal tubular cells showed that the maturation of intracellular organelles followed the proliferation of the premature cells.