Abstract: Hexoestrol-treated rats' uteri were shown always to give successful experiments in the routine bioassay of pituitary (posterior lobe) extracts. Rats each given a subcutaneous dose of hexoestrol dipropionate 10 μg in 0.2 ml. of arachis oil, 23-48 hours before use, were found to be at either proestrus or estrus stage. In selecting a suibtable rat for pituitary assay, this method is therefore preferred to vaginal smears method as authors had suggested in a previous paper.