There is increasing concern in the media, among politicians and within environmental organizations, about the effects of chemicals in our environment that affect the endocrine systems of wildlife and humans. At its simplest these are referred to as `gender-bending' chemicals or `environmental estrogens'. The chemicals in question (pesticides, PCBs, plasticizers, petrochemicals, and a variety of industrial chemicals) have been known to decrease human sperm counts, cause fish to `change sex', and increase male genital abnormalities. A great deal of pressure has been placed on environmental protection agencies to devise regulatory tests for the effects of these chemicals and to require limitations on their manufacture and release. Fish are increasingly recognized as an excellent model for such tests, in that the aquatic environment may provide early warnings of the effects that these chemicals will have on human health. In addition, the large number of eggs which fish produce provides an excellent model to examine the effects on female fertility. Endocrine Disruption in Fish provides a simple yet extensive background to the field of fish endocrinology in order to assist those toxicologists who have a limited background in either mammalian or fish endocrinology. It shows that environmental estrogens do not simply affect male reproductive potential, but that they may equally well affect the female. It is possible that these chemicals may have a complex effect on the brain, hypothalamus, pituitary, gonad and liver of both sexes. There are many other chemicals which affect both the gonads and other parts of the endocrine system to cause decreased fertility, abnormal sexual differentiation and behavior, decreased response to stress, immune deficiency or altered basal metabolism. Many thousand man-made chemicals are released into the environment, but very few have been tested for endocrine disrupting activity. This book covers only the minute fraction for which evidence has been collected.