The problem of progesterone metabolism provides biochemists with a subject of great interest and importance and one which will not be readily solved. Studies on excreta and on blood, especially investigations with isotopically labelled steroid; perfusion experiments and other in vitro techniques will all play their part in providing information. Until we have gained some insight into the biochemical mechanisms involved in progesterone metabolism we cannot hope to understand much about the causes of the variations which occur in the metabolism of this steroid in different species and in health and disease.