In order to examine to what extent adrenergic mechanism contributes to the urethral pressure in patients with benign prostatic hypertrophy, changes in the intraurethral pressure in the prostatic zone were measured in vivo by both the urethral pressure profile technique and the balloon method before and after administration of alpha-adrenergic stimulants and an alpha-adrenergic blocker. The effect of spinal anesthesia on the urethral pressure was also investigated. It is suggested that 40 per cent of the total urethral pressure in patients with benign prostatic hypertrophy is due to alpha-adrenergic tone, and the remaining 53 per cent is due to static pressure resulting from the hypertrophied prostatic bulk. The in vitro study indicates that the increase in urethral pressure and contraction of the prostate, prostatic capsule and prostatic urethra.