1. The effects of insulin treatment on in vivo and in vitro urinary bladder function in streptozotocin-diabetic rats were investigated. 2. Diabetes of 2 months duration resulted in decreases in body weight and increases in fluid consumption, urine volume, frequency of micturition, and average volume per micturition; effects which were prevented by insulin treatment. 3. Insulin treatment also prevented the increases in contractile responses of bladder body strips from diabetic rats to nerve stimulation, ATP, and bethanechol. 4. Diabetes of 4 months duration also resulted in decreases in body weight, and increases in fluid consumption, urine volume, frequency of micturition, and average volume per micturition, effects which were reversed by insulin treatment for the final 2 months of the study. 5. Insulin treatment reversed the increases in contractile responses of bladder body strips from diabetic rats to nerve stimulation, ATP, and bethanechol. 6. The data indicate that the effects of streptozotocin-induced diabetes on urinary bladder function are both prevented and reversed by insulin treatment.