Porphyromonas gingivalis is one of the oral microorganisms associated with human chronic periodontitis. The purpose of this study is to determine the role of the receptor activator of nuclear factor-κB ligand (RANKL) in P. gingivalis infection-associated periodontal bone resorption. Inbred female Rowett rats were infected orally on four consecutive days (days 0 to 3) with 1 × 10⁹ P. gingivalis bacteria (strain ATCC 33277). Separate groups of rats also received an injection of anti-RANKL antibody, osteoprotegerin fusion protein (OPG-Fc), or a control fusion protein (L6-Fc) into gingival papillae in addition to P. gingivalis infection. Robust serum IgG and salivary IgA antibody (P < 0.01) and T cell proliferation (P < 0.05) responses to P. gingivalis were detected at day 7 and peaked at day 28 in P. gingivalis-infected rats. Both the concentration of soluble RANKL (sRANKL) in rat gingival tissues (P < 0.01) and periodontal bone resorption (P < 0.05) were significantly elevated at day 28 in the P. gingivalis-infected group compared to levels in the uninfected group. Correspondingly, RANKL-expressing T and B cells in rat gingival tissues were significantly increased at day 28 in the P. gingivalis-infected group compared to the levels in the uninfected group (P < 0.01). Injection of anti-RANKL antibody (P < 0.05) or OPG-Fc (P < 0.01), but not L6-Fc, into rat gingival papillae after P. gingivalis infection resulted in significantly reduced periodontal bone resorption. This study suggests that P. gingivalis infection-associated periodontal bone resorption is RANKL dependent and is accompanied by increased local infiltration of RANKL-expressing T and B cells.