This reference discusses the important role of steroids and neuropeptides in the regulation of behavior. The guiding principle behind the discussion is the concept of using good model animal systems to help us to understand how hormones influence the brain. The book emphasizes that steroids and peptides or neuropeptides affect behavior by acting directly on the brain, and that common neural circuits underlie a variety of different central motive states. The first chapter focuses on developmental periods and sexually dimorphic behaviors; the second discusses sodium and water appetite, and ingestion; the third deals with appetite, food selection and ingestion. The fourth chapter examines how hormones influence parental behavior; the fifth is on fear and stress. The last chapter deals with biological clocks and endogenous rhythms.