The effects of five administrations (3- to 4-day intervals) of morphine (MOR: 10 and 20 mg/kg, s.c.) alone, MK-801 (dizocilpine: 0.03, 0.1, 0.3 and 1 mg/kg, i.p.) alone, and combinations of MOR with MK-801 on the ambulation in mice were investigated. MK-801 at 0.3 and 1 mg/kg, but not at 0.03 and 0.1 mg/kg, significantly increased the ambulation of mice. Although the mice given repeated administrations of MK-801 (0.3 and 1 mg/kg) exhibited enhancement and reduction, respectively, in the ambulation-increasing effect of the individual doses, they showed significantly higher sensitivity than the saline-treated mice to the challenge with MOR (10 mg/kg). The repeated administrations of MOR (10 and 20 mg/kg) induced a progressive enhancement of the ambulation-increasing effect. The mice repeatedly given MOR (10 mg/kg) exhibited significant increase in the sensitivity to MK-801 (0.03-0.3 mg/kg). The coadministrations of MOR with MK-801 intensified the ambulation-increasing effect, and repeated coadministrations induced progressive enhancement of the effect, except for the combinations of MOR (10 or 20 mg/kg) with MK-801 (1 mg/kg). However, the induction of MOR sensitization was not modified by any doses of MK-801, except for the case of combination of MOR (20 mg/kg) with MK-801 (1 mg/kg) which was highly toxic (i.e., eliciting death or a moribund condition). On the other hand, simultaneous treatment with SCH 23390 (0.05 mg/kg, s.c.) or nemonapride (0.05 mg/kg, s.c.), or 4-hr pretreatment with reserpine (1 mg/kg, s.c.) strongly, and 4-hr pretreatment with alpha-methyl-p-tyrosine (200 mg/kg, i.p.) partially reduced the ambulation-increasing effect of both MOR (10 mg/kg) and MK-801 (0.3 mg/kg). Simultaneous treatment with naloxone (1 mg/kg, sc) selectively reduced the effect of MOR. However, simultaneous treatment with apomorphine (0.1 mg/kg, s.c.) did not modify the effects of either drug. These results suggest that the characteristics of the ambulation-increasing effects of MOR and MK-801 are similar to each other, and that the repeated treatments with MK-801 induce a cross-sensitization to MOR and vice versa.