Using an instrumented spatial linkage, a method for measuring intervertebral motion in vivo was developed and used on six dogs. The segmental motion was recorded as the animals were exercised in routine functions. The standing posture was found to be a repeatable position. During walking, the average excursion between opposing facets was 3.4 +/- 1.3 mm, as the L2-L3 motion segment moved into 2.3 degrees of kyphosis with respect to the standing position. This method has the ability of measuring facet motion (+/- 0.7 mm), vertebral body motion (+/- 0.5 mm), and vertebral body rotations (+/- 0.6 degrees) with suitable accuracy such that it is a useful tool in documenting the in vivo response of a motion segment to surgical procedures.