The experimental model of inflammatory arthritis in the rabbit has been used to study the possible origin of the apparent synovial lining cell hyperplasia. A small amount of 3H-thymidine was injected into the joints and the fate of the labelled cells investigated by autoradiography. At times up to 24 h after the injection, most of the labelled cells occurred in the sub-lining region; this proportionality was reversed when the joints were sampled at later times. These results indicate that at least much of the increase of synovial lining cells may be derived from cells from deeper in the synovial tissue which move to the synovial surface.