Oral doses of 0, 1, 4, 5, 10, or 20 ml of Prudhoe Bay crude oil/kg body weight/day were given to herring gull and Atlantic puffin nestlings for 5 to 7 consecutive days. Gulls defecated substantial amounts of oil within 10 to 15 minutes after receiving a dose. Clinical signs and lesions occurred only in birds given greater than or equal to 10 ml oil/kg body weight/day. Gulls consumed less food and lost weight. Two categories of lesions were observed: those considered secondary to a primary toxic hemolytic disease, and those considered nonspecific reactions to stress. The former included phagocytosis of degenerate erythrocytes in the liver and spleen, hemoglobin resorption droplets in renal proximal tubule cells, and erythroid hyperplasia in the bone marrow; the latter included lymphocyte depletion in primary lymphoid tissues, an increase in heterophil: lymphocyte ratio in peripheral blood, lipid depletion and necrosis in adrenal steroidogenic cells, and an increased prevalence and severity of lesions in the bursa of Fabricius. These findings indicated that the primary target of oil toxicity was the peripheral red blood cell, but that significant stress-related lesions were also associated with ingestion of oil.