Endrin is an organochlorine alicyclic pesticide first introduced into the United States in 1951. Endrin has occasionally been found in drinking water and in food; measurable levels of endrin have not been detected in adipose tissue or the blood of the general population. Endrin is absorbed through the skin, by the lungs and by the gut, but no quantitative rates are known. Animals, birds and humans who have been exposed to large amounts of endrin have shown residues. In all warm-blooded species studied thus far, endrin is quickly metabolized and its metabolites quickly eliminated. Endrin deposition in tissues, especially fat, does occur at high doses in experimental animals and in birds. Residues have been detected in liver, brain, kidneys and fat. Endrin has a weighted average bioconcentration factor (BCF) of 3970 for the edible portion of all freshwater and estuarine fish and shellfish consumed by United States residents.