Abstract: Uterine spontaneous contraction and the irregular responses of uteri towards same doses of pituitary extract were very often encountered in the uterus method for pituitary bioassay.The rate of successful experiments was variable.It was found that uteri obtained from rats in proestrus stage gave much better results than those in either diestrus or estrus.Experiments employing ovarietomised rats treated or untreated with hexoestrol dipropionate also showed that uteri from rats in artificial diestrus gave disappointing results but uteri in artificial proestrus were as good as those in normal proestrus(Table Ⅰ). Table Ⅰ.Rate of Successful Experiments of Pituitary Assay Using Rat Uterus Method on Uteri in Different Stages of Estrous Cycle Rats(120—230g) Groups No.of Animals Used Rate of Successful Experiments Normal Diestrus 10 2/10 Proestrus 10 10/10 Estrus 10 6/10 Ovariectomised Treated with Hexoestrol dipropionate(artificial proestrus) 5 5/5 Untreated(artificial diestrus) 5 0/5 Reports on using diestrus uteri in rat uterus method for pituitary assay in previous literature are therefore proved to be not well founded. The possibility and problems involved in employing artificial proestrus uteri in pituitary assay are discussed.