The use of Recombinant Inbred mouse Strains (RIS) to derive information about the complexity of the genetic architecture underlying various traits is increasing in popularity. Behaviors measured to index sensitivity to drug effects and vulnerability to drug abuse are considered here. Potential uses of RIS are identification of major gene effects, mapping of traits to particular chromosomal sites, determining genetic correlations between characters, and identifying behaviorally extreme genotypes. This approach has led to identification of a major gene moderating alcohol acceptance in mice and has revealed a more complex polygenic system influencing morphine consumption.