Morphine ingestion: genetic control in mice.

C57BL/6J mice will drink large amounts of, and display a highly positive preference for, morphine sulfate when it is dissolved in an aqueous solution of sodium saccharin. In identical test situations DBA/2J mice will drink very little of, and display a strong avoidance toward, the morphine-saccharin solution. This clear separation between morphine-accepting and morphine-rejecting animals within a single species combined with a quick and simple method of inducing high levels of morphine ingestion could facilitate the discovery of causal factors in opiate addiction.