Two strains of inbred mice, C57BL/6J and DBA/2J, were tested for saccharin–morphine and saccharin–cocaine preference using a two-bottle, 24-h drug-control solution paradigm. Quinine–saccharin solutions were used to control for strain-related differences in preference for sweetened alkaloid solutions. The C57BL/6J mice consumed significantly more morphine than the DBA/2J mice and had higher cocaine preference scores. Morphine and cocaine preference scores of individual mice were significantly correlated in the C57BL/6J strain.