Abstract
1. Intrapituitary infusions of extracts made from the median eminence of various species at different ages were found to excite secretion of gonadotrophic hormone as shown by consequent ovulation, presumably due to their content of luteinizing hormone-releasing factor (LRF).2. Extracts from very immature animals (e.g. new-born rabbits and rats; 1-2-month-old calves) were found to be highly active with regard to LRF content.3. Extracts from foetal rabbits at 20 days of gestation showed no evidence of containing LRF.4. It is suggested that (a) LRF may have a trophic action upon the anterior pituitary gland during the pre-puberal phase of life, and that (b) the first formation of LRF coincides with the development of the primary plexus of the pituitary portal system.