A number of industrial oils were tested for their toxic effects on waterfowl. All oils were able to cause lipid pneumonia, gastrointestinal irritation, fatty livers, and adrenal cortical hyperplasia when fed to ducks in single doses by stomach tube. Feeding of a cutting oil and a diesel oil also resulted in acinar atrophy of the pancreas. The diesel oil and a fuel oil produced toxic nephrosis in a number of animals. Feeding the cutting oil produced a definite inhibition of cholinesterase activity while the diesel oil depressed cholinesterase activity only slightly. Approximate LDso values were determined for a number of oils under different environmental conditions. Gross examination of a series of 41 ducks which had been killed by oil pollution in the wild showed, at autopsy, changes similar to those encountered in the experimentally fed ducks. It was concluded that the toxicity of polluting oils is a definite factor in the observed mortalities due to oil pollution. Oil pollution has become a serious hazard to waterfowl during the last 30 years. The extent of waterfowl mortalities due to oil pollution has been reviewed by Hawkes (1961) and Erickson (1963). These reviews indicate that many thousands of birds die annually as a result of oil pollution. Two major factors appear to be involved in these deaths: the external oiling of the ducks and the actual ingestion of the oil. Richardson (1956) and Giles and Livingston (1960:299) have suggested that oil may be ingested by waterfowl during preening and may have toxic effects. Hartung (1963) demonstrated experimentally, using isotope-labeled oils, that oiled ducks ingest significant quantities of oil in preening their plumage. The present study was undertaken to investigate the toxic effects of oils on ducks. METHODS AND MATERIALS The pathological and physiological effects of ingested oils were evaluated by feeding oils to wild-trapped and domesticated ducks. Oils were fed undiluted as single doses by stomach tube. The oils utilized in these experimental feedings were: a light fuel oil (No. 1) containing less than 1 percent phenolic compounds as anti-oxidants; a diesel oil containing less than 1 percent organically bound phosphorus, phenols, and other unidentified additives; a simple, sulfuretted, low additive SAE 10W motor lubricating oil; a sulferetted SAE 10-W-30 motor lubricating oil with high detergent content; a high pressure cutting oil additive containing 30 percent chlorine and 10 percent phosphorus in organically bound form; and a cutting oil formulated with 10 percent of this additive, 10 percent triglycerides, and 80 percent mineral oil. Tissues for histological examination were collected immediately after sacrifice by decapitation, and fixed in buffered 10 percent formalin. In those cases where early histological examinations indicated specific organ damage, appropriate biochemical tests were made in subsequent experiments to try to determine the extent of alteration in function of these organs. Blood for analytical procedures was obtained by heart puncture or from the metatarsal vein, depending upon the amount of blood required. Hemoglobin was analyzed spectrophotometrically as oxyhemo1 Study supported by the National Science Foundation, National Wildlife Federation, and the Department of Wildlife Management, University of Michigan.