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Incidence of endocrine disruption in wild fish species has been documented globally and is well characterised in the UK, where the occurrence of intersex in roach (Rutilus rutilus) is widespread. Although this has been associated with concentrations of steroid oestrogens, research indicates that anti-androgenic chemicals may also play a role in inducing these effects. Anti-androgenic activity is commonly detected in wastewater treatment works effluents and some receiving waters, but the chemicals responsible remain largely uncharacterised. This thesis aimed to identify environmental anti-androgens in UK and South Australian catchments and to produce environmentally relevant exposures to assess their potential impacts on sexual disruption in fish, alone and in combination with steroid oestrogens. By using hydrological modelling techniques, pharmaceuticals with an anti-androgenic mode of action were predicted to occur in the ng/L concentration range in UK and South Australian wastewater treatment works effluents and river catchments. This work included analysis of future trends in environmental concentrations of the pharmaceuticals and the steroid oestrogens in these catchments. Modest increases in concentrations by 2050 were predicted in the absence of mitigation, which could increase in the risk posed to fish health by the steroid oestrogens in the future. The effects of the predicted concentrations of two pharmaceuticals, bicalutamide and cyproterone acetate, were then assessed in fathead minnows (Pimephales promelas) and Japanese medaka (Oryzias latipes) based on the UK modelling for the present day. These concentrations did not contribute to endpoints characteristic of sexual disruption, alone or in combination with steroid oestrogens. However, the results did support an environmental role for the steroid oestrogens in intersex induction. Concurrently, effect directed analysis identified some highly potent anti-androgens, such as triclosan and pyrene, in wastewater treatment works effluents from the UK. However, they are likely to make a minor contribution to overall anti-androgenic activity due to their low concentrations. Consequently, more work is required to identify the causes of this activity in the environment and its implications for wild fish health.