The role of serum glucose levels on the sensitivity to the antinociceptive effect of morphine in streptozotocin-induced diabetic mice and C57BL/KsJ db/db mice were examined. The sensitivity to the antinociceptive effect of morphine was significantly reduced in streptozotocin-induced diabetic mice as compared with age-matched nondiabetic mice. Pretreatment with insulin (3 U/kg, s.c.) significantly reduced the serum glucose levels of streptozotocin-induced diabetic mice as compared with those of untreated diabetic mice. However, post-drug (morphine) tail-flick latency was not affected by pretreatment with insulin. The antinociceptive effect of morphine was also significantly reduced in C57BL/KsJ-db/db mice as compared with age-matched control mice. When CS-045 was administered to C57BL/KsJ-db/db mice, the serum glucose levels were significantly reduced. There was no significant difference in the antinociceptive effect of morphine between CS-045-treated C57BL/KsJ-db/db mice and C57BL/KsJ-db/++ mice. Adoptive transfer of supernatant of the spleen cell homogenate from C57BL/KsJ-db/db mice to naive ICR mice had no significant effect on the recipients' antinociceptive sensitivities to s.c. morphine. These findings support the our previous suggestion that some factor(s) derived from spleen mononuclear cells is the prime factor involving the insulin-insensitive mechanisms for the reduction of mu-opioid agonist-induced antinociception during the severe stages of diabetes.