Abstract: A method for the bioassay of pituitary(Posterior lobe)preparations using the mouse uterus in poestrus was reported. A pituitary preparation of known strength(1 I.U./ml)was assayed for 13 times with mice uteri.The mean estimated value was 1.034 I.U./ml.The coefficient of variation being 9.40%. On the same preparation four assays using rats uteri were carried out, the average result being 1.019 I.U./ml.It is therefore considered that the mice uterus method can be adopted as a convenient and economical device for the bioassay of pituitary(posterior lobe).