Lower urinary tract symptoms (LUTS), including frequency, urgency, incomplete voiding, slow stream are common in both men and women with advancing age. The most common cause for LUTS in aging men is benign prostatic hyperplasia (BPH). Some studies have also revealed an inverse association of serum testosterone levels with LUTS; however the underlying mechanisms by which gonadal hormones affect the LUT have not been clarified. In the present study, we examined the effect of orchiectomy and testosterone replacement on lower urinary tract function in adult male Sprague-Dawley rats. Six weeks after bilateral orchiectomy or sham operations and 3 weeks after injection of long acting testosterone undecanoate (100 mg/kg i.m, transvesical cystometry and external urethral sphincter electromyogram (EUS EMG) recordings were performed under urethane anesthesia. The micturition reflex was elicited in both sham and orchiectomized animals. In orchiectomized rats volume threshold for inducing micturition decreased by 47.6%; however contraction amplitude, duration, and voiding efficiency were similar in sham and orchiectomized rats. The active period during EUS EMG bursting was lengthened during micturition in orchiectomized animals. Testosterone treatment which normalized plasma testosterone levels reversed these changes but also increased the duration of EUS EMG bursting (BD). Orchiectomy also reduced mean voiding flow rate estimated from the duration of BD, an effect that was not reversed by testosterone. The results indicate that orchiectomy affects both the active and passive properties of the bladder and urethra and that many but not all of the changes can be reversed by testosterone.