In the past year, some of the most exciting findings in the genetic investigation of mammalian behavior have been obtained through mapping and through gene manipulation studies in the mouse system. These include the localization of a gene for circadian periodicity in the mouse, gene knockouts of serotonin receptors, and the development of a transgenic model of Alzheimer's disease. The recent development of genetic maps covering the entire human genome and the implementation of new approaches to genetic analysis may now facilitate elucidation of complex behaviors in humans, particularly psychiatric disorders.