A mechanically-expandable instrument (bougie), especially useful for treating diseases in the urethra and the neck of the urinary bladder, comprises an adjustable-speed driving unit (20) and a probe (22) composed of two rods (64) and (66) pivotally connected together at one end thereof and attached to output sliders (54, 56) of the driving unit at their other ends. Each of the rods (64) and (66) has a semicircular cross section so that in an assembled state the probe has a complete circular cross section. Reciprocations of the sliders cause expansions and contractions of the probe, thereby to stretch urethral constrictions in the radial direction and to subject the urethra and its the surrounding tissue to massaging and vibrational actions which are extremely efficient for treating diseases of the urethra and the neck of the urinary bladder. Other embodiments of the rod employ cams, wedges, and threaded means for creating radial expansions and contractions of the probe.