A stretch test in vivo for living materials was newly developed and applied to the dog bladder. The stress-strain curve of the normal living bladder was a downward convex and rose very slowly. The curve showed hysteresis and speed-dependence. These results proved that the mechanical property of the urinary bladder was essentially viscoelastic. A computer analysis, based on the viscoelastic simulation model proposed by Glantz, was performed to evaluate the mechanical property of the bladder. Both proportional and exponential stiffness elevated in the extirpated bladder. In the denervated bladder, changes of the constants were not significant immediately after cutting the pelvic nerves; however, the proportional stiffness elevated remarkably two weeks after cutting. Compared with these abnormal bladders, the elastic constants of the normal living bladder were very low. The viscoelastic properties of the bladder and its low elastic constants seemed to account for the normal bladder function as a reservoir for urine.