Abstract We investigated whether enhanced spontaneous locomotor activity in streptozotocin (STZ)-induced diabetic mice is associated with serum glucose levels. Furthermore, the role of factor(s) derived from spleen cells on the enhanced spontaneous locomotor activity in diabetic mice was also examined. Serum glucose levels were significantly increased on day 4 after STZ administration and remained increased at 7, 14, 28 and 56 days after STZ administration. A significant increase in spontaneous locomotor activity in diabetic mice as compared to that in non-diabetic mice was observed 4, 7, 14 or 28 days after STZ treatment. The increase in spontaneous locomotor activity in diabetic mice was not observed when insulin was chronically administered to mice 3 to 7 days after administration of STZ. Splenectomized diabetic mice, which were operated upon 7 days after STZ administration, still had a significantly higher spontaneous locomotor activity than non-diabetic mice. In contrast, spontaneous locomotor activity in diabetic mice that were splenectomized either before or 3 days after administration of STZ returned to the levels in non-diabetic mice. The rate of dopamine (DA) turnover in the limbic forebrain of diabetic mice on day 14 was significantly higher than that in age-matched non-diabetic. Although the rate of DA turnover in the diabetic mice that were splenectomized 7 days after STZ administration was significantly higher than that in non-diabetic mice, an increased the rate of DA turnover was not observed when mice were splenectomized either before or 3 days after administration of STZ. These results suggest that the lasting hyperglycemia may be responsible for their hyperlocomotion in diabetic mice within 14 days sfter STZ treatment, perhaps due to an increase in dopamine release in mesolimbic dopamine systems. Furthermore, factor(s) derived from spleen cells of diabetic mice during the incipient stages of diabetes may play a role in the enhancement of dopaminergic neurotransmission.