Pathohistologic changes in various organs of eels kept for 14 days in water tanks containing crude oil with and without various oil detergents were studied. The results were: 1. Eels kept in water containing crude oil showed marked degeneration of proximal renal tubules. There was some regeneration of tubular epithelial cells and an increase in hemosiderin-like substance in the interstitial tissue of the kidney and spleen, suggesting accelerated hemolysis. The increased deposition of hemosiderin-like substance seemed to be due to hemolytic activity of the benzine, toluene and xylene contained in crude oil. No marked changes were seen in the liver, gills, muscle, intestine or other organs examined. 2. Eels exposed to oil detergents also showed kidney damage, tubular necrosis and regeneration. Increased deposition of hemosiderin-like granules was evident. The activated hemolysis seemed to be due to cytolytic activity of the surface active agents. 3. In the cases where crude oil was given with oil detergents pathologic changes were most marked. Severe degeneration of the proximal renal tubules was observed with poor regeneration of the epithelial cells. These severe changes may be due to accelerated migration of crude oil components into the fish body by oil detergents rather than to synergistic effects of the agents and crude oil. 4. Of the substances contained in crude oil those that are known to penetrate into the living body are, in the mammal, all liver-damaging but not kidney damaging. The same substances, however, may cause kidney damage in fish, because the function of fish kidneys is very different from that of mammal kidney.