We have already reported that morphine pretreated rats prefered morphine-admixed food during choice trials with the two cup method. In the present work, we utilized both the five and two cup methods and observed the preference for morphine in rats forcedly pretreated with the drug and the increasing rate of preference for the drug in rats where the feeding time was limited. In morphine pretreated rats, preference rate for morphine was 61.2 +/- 3.0% with the five cup method and 61.8 +/- 3.3% with the two cup method during the choice trials. In rats that were limitedly treated with morphine, each preference rate for morphine during choice trials rapidly increased in the five cup method, i.e. 5.5 leads to 14.4 leads to 31.7 leads to 43.6 leads to 61.2%, and the preference rate for morphine stabilized at approximately 60 percent. Findings with the two cup method were similar. After the preference for morphine was stabilized at the 60 percent level, morphine was given subcutaneously and it was found that the preference rate was dependent on the dose injected. When the drug-admixed concentration was changed from 1 mg/g food to 0.5 and 2 mg/g food, the preference rate changed in parallel with the concentration. When the number of food cups containing morphine-admixed food was changed from 1/5 to 2/5, 3/5 and 4/5 food cup, the preference rate was not effected. These studies clearly demonstrate drug-seeking behavior in rats. In the process of preference for morphine, morphine treatment enhances spontaneous intake of morphine-admixed food.