[The quantitative evaluation of the preference for morphine by rats (author's transl)]

We examined the quantitative evaluation of preference for morphine by assessing the conflict behavior between positive motivation and negative motivation. The apparatus consisted of a single runway in which both the usual diet and morphine-admixed food were placed. For access to the latter, the rats were compelled to pull heavy weights which were connected to their necks. During the forced trials (only morphine-admixed food was given) and the choice trials (rats could select either food), the preference rates gradually increased and then became stable at levels of 60%. After these trials, experiments on conflict behavior were performed using several different weights. Assessment of relation between the intensity of the dependence on the drug and the degree of weight showed a good correlation. We conclude that the preference for a drug can be quantitatively measured by means of assessing conflict behavior.