The present study examined behavior of streptozotocin-diabetic mice in Porsolt's swim test, a putative animal model of depression, in the holeboard test of exploration and locomotor activity, in the plus maze test of anxiety, and in the resident-intruder paradigm of aggression. Two weeks after an IP injection of 200 mg/kg streptozotocin, which caused a 20% weight loss and increased fluid consumption and urination, male NIH Swiss mice were found to show lengthened duration of immobility in the swim test. One week of insulin treatment (0.1 IU/g/day) partially antagonized this change. The locomotor activity scores in the streptozotocin-treated mice were lower in the holeboard but higher in the plus maze than in the controls; therefore, the lengthened immobility was not likely to be due to a general motor impairment. No significant changes in the time spent in social interaction or aggressive behavior were found in the streptozotocin-treated mice. The results indicate that streptozotocin-treated mice show lengthened immobility in the swim test.