We observed the peripatellar synovial membranes of normal and betamethasone-treated rats with a scanning electron microscope (SEM), light microscope (LM) and transmission electron microscope (TEM). On the basis of SEM findings, the peripatellar synovial membrane is divided into two distinct regions; an upper, smaller, round, non-folded region which is covered with small excrescences, and a lower, larger, horseshoe-shaped region which has numerous folds consisting of cobblestone-like oval cells. These SEM findings were well confirmed by parallel LM and TEM observations of both regions. The synovial membrane of rats treated with intra-articular injections of betamethasone, an anti-inflammatory agent for rheumatoid arthritis, showed cell-sized excrescences on the surface of the non-folded regions. Under LM and TEM, the synovial membrane of treated rats showed an expansion of the intercellular spaces with collagen fibers, swelling of vacuoles in the cytoplasm in the type M synoviocytes, and increases in the number and elongation of microvilli of the type M synoviocytes. These observations suggest that repeated betamethasone injection therapy may result in unnecessary damage to the synovial membrane.