Achilles tendon reflex time was measured in normal subjects (1,837 femates and 7,937 males). Values of half relaxation time showed a clear dependency on both age and sex. The reflex becomes slower with advancing age and was consistently slower in females. The results redefine the normal range of the Achilles tendon reflex. Regression lines of relaxation time on age were fitted for each sex separately. Lines representing linear regression, log-linear regression or quadratic regression were inferior to those described by linear splines. The gradual increase of Achilles tendon relaxation time with age was not due to unrecognized hypothyroidism or peripheral neuropathy. It is suggested that one cause of the progressive increment is diminution of sympathetic tone with age, and changes in the muscles themselves may also contribute.