Involvement of supraspinal and spinal mu opioid receptors in the development degree of morphine dependence was estimated by the ED50 values of naloxone (s.c.) required to precipitate withdrawal jumping and diarrhea 72 hr after morphine-pellet implantation. beta-FNA was administered 4 times (24 hr before, just before, 24 and 48 hr after morphine-pellet implantation) by the i.c.v. or i.t. route. beta-FNA (both i.c.v. and i.t. routes) significantly increased the ED50 values of naloxone for jumping and diarrhea and the increase in the ED50 value of naloxone for jumping was much greater than that for diarrhea. I.c.v. administered beta-FNA was more potent in increasing the ED50 value of naloxone for jumping than i.t. administered beta-FNA. On the other hand, i.c.v. administered beta-FNA was equipotent with i.t. administered beta-FNA in increasing the ED50 value of naloxone for diarrhea. These results suggest that both supraspinal and spinal mu opioid receptors are involved in the development of morphine dependence and that supraspinal mu receptors play a more important role than spinal mu receptors in withdrawal jumping which may reflect an excitation of the central nervous system whereas supraspinal and spinal mu receptors have similar importance in withdrawal diarrhea which may reflect an abnormal function of the autonomic nervous system.