Bone loss represents a major unsolved problem in rheumatoid arthritis (RA). The receptor activator of nuclear factor-kappaB ligand (RANKL) is essential for the development and activation of osteoclasts, which are key mediators of bone erosions. This study was performed to determine temporal and spatial expression of RANKL compared with the potentially destructive cytokine interleukin-1beta (IL-1beta), related to progression of synovitis and joint destruction in collagen-induced arthritis (CIA), a model of RA. CIA was induced in dark agouti (DA) rats, and tissue specimens were obtained for immunohistochemical analyses at various time points before and after disease onset. Arthritis was monitored visually, and joint pathology was examined histologically. No disease-preceding expression of RANKL was detected. However, a marked increase of both RANKL- and IL-1beta-expressing cells correlated with the progression of synovial inflammation and clinical disease severity. Abundant and concomitant expression of these cytokines was detected at sites of bone erosion, where a colocalization by osteoclast-like multinuclear tartrate-resistant acid phosphatase (TRAP)+ cells was noted. In contrast to the paucity of RANKL expression in cartilage, an abundant expression of IL-1beta was demonstrated, particularly in superficial cartilage layers. These data support the hypothesis that RANKL and IL-1beta are central contributors to joint destruction in CIA.