Muscles in the limbs change with age, but the aging process of urethral muscles is unknown. Therefore, we compared smooth and striated muscle content in the female rat urethra in young (12 month) and old (32 month) animals, using immunochemical techniques. All the striated skeletal fibers at both ages contain slow myosin. Urethral diameter does not change with age (young, 1.44 +/- 0.08 mm; old, 1.46 +/- 0.10 mm, n = 5), nor does the external sphincter width (young, 0.088 +/- 0.016 mm; old, 0.080 +/- 0.017 mm, n = 5). Neither smooth nor skeletal muscle volume in the urethra is changed with age (skeletal: young, 20.72 +/- 2.94%; old, 19.95 +/- 2.35%. Smooth: young, 22.26 +/- 2.98%; old, 26.75 +/- 2.35%, n = 5). The external striated sphincter is separate and distinct from the pubococcygeal region of the levator ani muscle, but is closely apposed to another layer of longitudinally oriented fibers into the vaginal musculature. The morphometric analysis shows no difference in urethral architecture in aging female rats.