It is known that the CXBK inbred strain of mouse is deficient in mu1 opioid receptors, whereas the strain has a delta opioid receptor population that is less consistently altered. In the present study, we compared physical dependence on morphine between CXBK and C57BL/6 mice. Both strains of mice were treated with morphine-admixed food for 5 days. During the treatment, the two strains of mice showed no signs of toxicity. There was no significant difference in morphine intake during the treatment between CXBK and C57BL/6 mice. After the treatment, the withdrawal was precipitated by injecting naloxone (0.01-30 mg/kg, s.c.). CXBK mice showed weight loss, diarrhea and ptosis, but not jumping and body shakes after low dose of naloxone. Whereas, C57BL/6 mice showed weight loss, diarrhea, ptosis, body shakes and jumping. These results suggest that naloxone-precipitated weight loss, diarrhea and ptosis may be mediated by mu2 and/or delta opioid receptor, while naloxone-precipitated jumping and body shakes may be mediated by mu1 opioid receptors.