By means of a water plethysmograph, changes in venous arm volume were determined for 10 mm .-increments of venous pressure, from 20 mm. to 60 mm. Hg. The volume-pressure changes were used to calculate E, the coefficient of functional elasticity, of veins in vivo. The results were checked by measurements of E and M (co-efficient of linear stretch) on veins from autopsy material. The values of E are subject to wide individual variation and show a progressive fall with age. The decrease in E (increase in extensibility) with age is not due to a change in M as this is found to be the same in different age groups. The increased extensibility is thought to be due in part to a decreased production of tissue fluid in older subjects.