QTL (quantitative trait locus) mapping is an exciting new technique currently being applied to many quantitative traits. We review the animal models and molecular genetic techniques of QTL mapping with particular reference to ethanol-induced anesthesia (sleep time). Sleep time is an easily obtained measure of initial sensitivity to ethanol. We are using three-stage strategy to map QTLs causing the difference in sleep time between selected strains of mice. This project has been expedited by the use of polymorphic genetic markers (simple sequence repeats) which are amplifiable by the polymerase chain reaction. Over 115 strain distribution patterns have been characterized in the 27 LSxSS recombinant inbred strains. Provisional QTLs have been identified in the RI strains and will be confirmed or rejected in an F2 population of 1063 mice. A new statistical technique, interval mapping, allows the use of all polymorphic markers simultaneously to locate QTLs.