IBN-9511359 Axelson, John F. The development of brain structural and behavioral sexual dimorphisms is a topic of intense investigation, as are attempts to define the factors responsible for inducing the structural changes. However, little information about the different functions of brain dimorphic peptide modulatory systems have been forthcoming. In the present studies, Dr. John Axelson proposes to investigate how steroid hormones modulate brain development during critical pre- and post-natal periods. These studies will determine if modulation of the steroid hormone environment during early or late development influences not only behavioral patterns and memory, but also the amounts of information transmitting molecules in the brain. Dr. Axelson will be investigating the role of the vasopressin-containing neurons in the brain and whether this neuropeptide system is involved in sexually dimorphic development of social recognition memories. Further, it is proposed to examine the role of steroid hormones in the generation of dimorphic patterns of vasopressin expression as a basis for understanding their role in behavioral changes. The proposed studies will generate novel information about brain development and the corresponding behaviors that will enhance our understanding of human behavior and cognition.