In an earlier communication [Emmens, 1941] a method was described for the assay of oestrogens using intravaginal application in aqueous or 50% aqueous glycerol solutions. The potencies of oestrone, α-oestradiol and oestriol were found to be very similar when assayed intravaginally in spayed albino mice by a 2-injection technique, confirming the results of Mühlbock [1940], who used a 3-injection method. Mühlbock found oestrone to be about twice as potent as oestradiol and about three times as potent as oestriol. Emmens found that oestrone was nearly twice as potent as oestradiol and over three times as potent as oestriol, with the reservation that the exact potency depended on the level of response at which the comparison was made, since his dose-response lines were not parallel. This latter finding suggested that it would not be feasible to use the intravaginal route for exact work, but several factors have since led to a