Many tumours in endocrine-sensitive tissues, such as the breast and endometrium, are hormone-dependent and the hydrolysis of oestrone sulphate (EIS) to oestrone by oestrone sulphatase (EI-STS) is a major source of oestrogen in such tumours. Oestrone-3-O-sulphamate (EMATE) has been shown to be a potent EI-STS inhibitor in vitro, and in this study its ability to inhibit enzyme activity in vivo was examined. EMATE was initially administered to female rats for 7 days, after which liver EI-STS activity was measured. As EMATE also inhibits a related sulphatase in vitro, dehydroepiandrosterone sulphatase (DHA-STS), its effect on the activity of this enzyme in vivo was also investigated. DHA-STS has a pivotal role in regulating the synthesis of another steroid with potent oestrogenic properties, androstenediol. Administration of EMATE almost completely inhibited liver EI-STS (99%) and DHA-STS (99%) activities and was active when given by the oral or subcutaneous routes. After a single dose of EMATE or following the cessation of multiple doses for 10 days, liver EI-STS activity remained inhibited ( > 95%) for up to 7 and 10 days, respectively. Other compounds, such as 4-hydroxytamoxifen and the "pure" antioestrogen ICI 182,780, which are reported to inhibit EI-STS activity in vitro, did not inhibit activity in vivo. In a preliminary study, EMATE, when injected over a 12-day period, effectively reduced the growth of EIS-stimulated nitrosomethyl-urea-induced mammary tumours in ovariectomised rats and inhibited tumour sulphatase activity in treated animals.