Forty-three autopsy cases with neither history nor anatomic evidence of hepatobiliary or pancreatic diseases were submitted to morphometrical evaluation of the internal diameter of the common hepatic duct. At a cross section of the duct the perimeter length of the mucosal basement membrane was measured to calculate its diameter D to be obtained when it was stretched into a circle. The value of D was found to correlate significantly with age, with the upper rejection limit (5% level) increasingly elevated from 4.8 mm at 20 years to 7.5 mm at 80 years. This implies that the age of patient should be taken into account in judging clinically whether or not bile ducts actually are dilated. The density in the ductal wall of elastic fibers was also determined and was shown to increase as the senile dilatation progressed, a sign of adaptive reinforcement of wall tissue that is liable to gradual hysteresis under chronic tension load.