Physical dependence on orally administered loperamide hydrochloride (loperamide) was investigated in comparison with codeine phosphate (codeine) in mice and rats. Jumpings were produced in 50% or more of mice used by an intraperitoneal injection of nalorphine after the repeated administration of 60 mg/kg/2 d or more of loperamide and of 399 mg/kg/2 d or more of codeine, but in 10% after 10.5 to 45 mg/kg/2 d of loperamide. Dosing of 50 mg/kg/d of loperamide resulted in a partial substitution for morphine in morphine physically dependent rats, but dosing of 5 and 10 mg/kg/d did not produce significant activity. Codeine (s.c.) showed complete substitution activity for morphine. Significant withdrawal signs were not observed after abrupt withdrawal of loperamide or after the administration of narcotic antagonist instead of loperamide in the rat given chronic administration of loperamide at maintenance doses of 2.5 and 5 mg/kg×2/d, though the high dose (10 mg/kg×2/d) of loperamide as well as codeine (40 mg/kg×2/d) caused the definite withdrawal signs. Its withdrawal signs, however, were partly different from that in the rat given with codeine. Serum loperamide levels were 1336 and 155 ng/ml after repeated dosing of 60 mg/kg/2 d to mice and 10 mg/kg×2/d to rats, respectively. These levels in mice and rats were approximately 136 and 263 times, respectively, that after the single oral administration of the dose (0.1 and 1 mg/kg) effective in preventing diarrhea induced by castor oil. From these results, it was suggested that loperamide produced definite physical dependence similar to codeine if its high dose was given repeatedly to mice and rats, and that there was a large difference between the doses producing physical dependence and anti-diarrheal action.