Recombinant inbred (RI) mouse strains were developed primarily as a tool to detect and provisionally map major gene loci--those with effects large enough to cause a bimodal distribution in the trait of interest. This implied that progress toward gene mapping was possible only for gene loci accounting for at least half of the genetic variance. More recently, QTL (quantitative trait loci) approaches have been advanced that do not require bimodal distributions and are thus applicable to a much wider range of phenotypes. They offer the prospect of meaningful progress toward detecting and mapping minor as well as major gene loci affecting any trait of interest, provided there is a significant degree of genetic determination among the RI strains. This paper presents a review of RI gene mapping efforts concerning phenotypes related to drug abuse and presents new data for studies now in progress for nitrous oxide and acute ethanol withdrawal intensity. These two studies exemplify several strengths and limitations of the RI QTL approach.