The use of controlled dietary intake for administration of environmental oestrogens to laboratory animals is essential in assessing the real threat of natural exposure to these compounds. Although human and animal exposure to environmental chemicals is mostly through diet or water, administration of these chemicals to laboratory animals is usually via injection. In this study, in order to overcome the problem of unpalatability of high doses of chemicals, a wax and a powdered diet were designed. These diets were used as vehicles for administration of the environmental oestrogen octylphenol (OP) to non-pregnant, pregnant and lactating laboratory rats and the dose of dietary octylphenol required to induce oestrogenic effects in the reproductive tract of female animals was determined.