Quantitative trait loci mapping was used to identify the chromosomal location of genes which contribute to oral morphine preference (in a two-bottle choice paradigm) of C57BL/6J mice, compared to DBA/2J mice. An F2 intercross of these two strains (606 mice) was phenotyped for morphine preference and those mice demonstrating extreme values for morphine consumption (the highest and lowest 7.7%) were genotyped for 157 murine microsatellite polymorphisms. Maximum likelihood methods revealed three loci on murine chromosomes 1, 6 and 10 which are responsible for nearly 85% of the genetic variance observed between the two parental strains.