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The objectives of the present study were to measure oestrone derivatives [oestrone, oestrone sulphate (E~1~S) and oestrone glucuronide] in sow faeces by direct enzymeโlinked immunosorbent assay (ELISA) and to explore the changes of oestrone derivative profile in faeces of miniature pig for demonstrating the possibility of pregnancy diagnosis. Faecal samples (1.5โg) were put into 6โml of buffer, shaken and centrifuged. Then supernatant was added to the wells of multiโplate without extraction and incubated with antiโoestrone antibody and horseradish peroxidaseโlabelled oestrone. Standard solution was prepared at various concentrations of E~1~S. Sensitivity was estimated as 0.035โng/ml (0.14โng/g). Intraโ and interโassay coefficient variations were 3.5โ7.7% and 10.9โ15.3%, respectively. When 1โ5โng/ml E~1~S were added to a faecal solution, recovery rates ranged between 80.0 and 103.3%. There is a temporal increase in the E~1~S equivalent concentration of miniature pig faeces from day 25 to 31 after mating. From day 35 to 70, the E~1~S equivalent concentrations remained low. Thereafter its concentrations increased again towards farrowing. On day 27 and 29 after mating pregnancy diagnosis by the faecal E~1~S equivalent concentrations agreed with the results of farrowing (seven of seven animals). These results suggest that the present direct ELISA is practical and suitable as a routine assay for measuring the faecal concentration of oestrone derivatives and that this assay might be usable for pregnancy diagnosis in sows at day 27โ29.