The production of spermatozoa depends on environmental factors, such as climate, season, nutrition, etc., and on internal factors, such as general health and endocrine secretions. These latter secretions play a very important part in the production of germ cells, and a study of their influence on sperm production should be of great value in both human and veterinary medicine. The present experiment was planned to investigate the influence of oestrogens on the sperm production of normal adult rams. The synthetic oestrogen diethylstilboestrol was used throughout. MATERIAL AND METHODS Two Suffolk rams (nos. 84 and 86), 3 years old, were used in this experiment. They were kept in a field at the Animal Research Station, Huntingdon Road, Cambridge. The general management was the same as on any farm in south-east England. They were in fairly good condition, and their body weight was almost constant throughout this study. They have been used