The spontnaeous morphine intake ratio (M-SIR) under free access to morphine-admixed food and quinine-admixed food conditions was measured for 3 weeks in naive and morphine-experienced rats. In the case of morphine (0.5 mg/g of food) vs. quinine (0.5 mg/g of food), naive rats gradually increased M-SIR from 17% to 77%. Using a higher level of morphine- and quinine-admixed food (1 mg/g vs. 1 mg/g of food), M-SIR was more rapidly increased than that in the lower group. Thus while on the 10 approximately 60 mg/kg/day dose range, the M-SIR was gradually increased dose dependently in naive rats due mainly to the positive reinforcing properties of morphine. Morphine-experienced rats showed a significant increase in M-SIR for the first 4 days specifically as compared with naive rats. Morphine dependent rats thus obtained morphine in sufficient amounts to maintain dependent states only after the first 2 approximately 3 days. This choice behavior revealed the psychological aspects of morphine dependence in rats and the preference for morphine was also observed after withdrawal for more than 2 weeks as secondary abstinence syndrome.