Among the species investigated, the rate of spontaneous cancers of mammary glands is highest in humans and dogs (Hamilton, 1974; Owen, 1979). In addition, spontaneous mammary tumours are known to occur in strains of mice (Webster & Muller, 1994), rats (Russo et al. 1990; Sukumar, 1995) and cats (Hamilton, 1974; Kessler & von Bombard, 1997). Although both oestrogen and progesterone are known to be involved in normal mammary development as well as in the proliferative changes that occur during the oestrous or menstrual cycle, pregnancy and lactation (Going et al. 1988), only the role of oestradiol has been extensively investigated. The role of progesterone in mammary cancer is not understood and, although progesterone metabolism is known to occur in mammary tissue, the potential autocrine/paracrine role of the in situ metabolites has only recently begun to be explored in human breast tissues and cell lines (Wiebe et al. 2000).