The assessment of risks to the aquatic environment in the European Union is generally based on a comparison of Predicted Environmental Concentrations (PEC) with Predicted No Effect Concentrations (PNEC) for surrogate, or “representative”, organisms of the receiving waters. Such risk assessments are required for new and priority existing chemicals, pesticides and, in the near future, biocides; they are dependent on robust in vivo test data. Current strategies for ecological risk assessment were not designed to assess the risk of endocrine disrupters. The selection of suitable fish species and practical in vivo end points for determining endocrine disruption in fish are discussed, including the adaptation of some existing guidelines. This paper is partly based on a series of experiments conducted our laboratory to look at the effects of a model alkyl-phenol (4-tert-pentylphenol), an industrial chemical intermediate, acting as a pseudo-estrogen on an all-male population of the common carp, Cyprinus carpio. Exposure to the test substance occurred at critical life stages for endocrine disruption. Biochemical parameters as well as histological parameters were applied, and their suitability to be used in ecological risk assessment is discussed.