Summary. Oestradiol-17β and conjugated oestrone, oestradiol-17β and oestradiol-17α were measured in peripheral plasma of heifers treated with PMSG/PGF-2α to induce superovulation. Changes in the concentration of each hormone were synchronous, the highest level being near oestrus. For a given number of ovulations the hormone with the highest concentration was total oestradiol-17α, then came total oestrone, total oestradiol-17β and oestradiol-17β. For each oestrogen, the maximum preovulatory concentration measured was significantly correlated with the number of ovulations; the regression line for total oestradiol-17α was twice as steep as that for oestradiol-17β. It is concluded that in animals treated to induce superovulation assay of total oestradiol-17α gives a better indication of the number of follicles induced to ovulate than does the more conventional assay of oestradiol-17β.