AIMS: Aging is an obvious risk factor for detrusor underactivity. We investigated the effects of aging on bladder function in spontaneously hypertensive rats. MAIN METHODS: Male spontaneously hypertensive rats and Wistar Kyoto rats (used as normotensive controls) at the ages of 18, 36, 54, or 72 weeks were used. Bladder weight, blood pressure, bladder blood flow, and urodynamic and renal parameters were measured. Additionally, detrusor thickness and renal histology were evaluated. KEY FINDINGS: In spontaneously hypertensive rats, significant increases were observed in bladder weight/body weight ratio, blood pressure, detrusor thickness, intercontraction interval, urine output, serum creatinine, and renal glomerular and tubular scores, and decreases in bladder blood flow and urine osmolality at 72 weeks as compared to those at 18 weeks. In spontaneously hypertensive rats, significant increases were observed in single voided volume, post voiding residual urine volume, and bladder capacity, with decrease in voiding efficiency were observed at 54 or 72 weeks than at 18 weeks. However, there were no significant differences in blood pressure, urodynamic and renal parameters, detrusor thickness and renal histology among Wistar Kyoto rats of different ages. SIGNIFICANCE: In spontaneously hypertensive rats, aging induces significant increases in blood pressure, single voided volume, post voiding residual urine volume, intercontraction intervals and urine output, and decreases in voiding efficiency and bladder blood flow indicative of detrusor underactivity. Aging-related severe hypertension could induce voiding dysfunction such as detrusor underactivity via severe bladder ischemia and polyuria. Aged spontaneously hypertensive rats may be useful animal models for detrusor underactivity.