The interlaminar ligament between the neural arches of L3 and L4 vertebrae of 10 post mortem subjects, aged 13–79 yr, was subjected to tension tests in Ringer solution. The ligament was almost perfectly elastic in behavior. This property could be due to the high contents of elastic fibers in the ligament as the proportion of elastic to collagen fibers, as obtained by biochemical assay in four ligaments was 2:1. The ligament prestresses the disc by a force ranging from 1500 g in the young to 400 g in the aged. It accounts for the intra-discal pressure of about 0.70 kg/cm² as found by pressure measurements in autopsy spines. The elasticity of the ligament prevents it from protruding into the spinal canal in backward bending. In forward bending it exerts a momentum of up to about 30 kg cm, which is small load compared to the load on the disc in the living man. The ligament exhibits non-linear viscoelastic properties, but the time dependent behavior is less significant that in skin, tendon or costal cartilage. Values for stress and strain at rupture ranged from 100 kg/cm² at 70 per cent in the young to 20 kg/cm² at 30 per cent in the elderly, respectively with significant age correlation for stress. The Modulus of Elasticity at rupture decreased significantly with age from 1.000 kg/cm² in the young to 200 kg/cm² in the aged.