Objective: Control of subgingival plaque is of paramount importance in the treatment of periodontal diseases. The role of subgingival sustained-release antibiotic therapy needs elucidation. A single-blind clinical trial was carried out in 13 patients with adult periodontitis to compare the effects of subgingival application of metronidazole dental gel with those of subgingival scaling. Method and materials: A split-mouth design was used so that each patient received all treatments simultaneously. Randomly selected quadrants were treated with application of 25% metronidazole gel, subgingival scaling, or a combination of scaling and gel application. The remaining quadrant in each patient was left untrated as a control. Results: All three treatments were effective in significantly reducing Plaque Index, Gingival Index, and bleeding on probing over the 14-week observation period. No statistically significant differences were found between scaling alone and bomined treatment. Scaling and combined treatment were better than metronidazole. Metronidaz ole produced transient effects, best noted during the first 4 weeks after treatment. No additive effect o f metronidazole was noted in the combined treatment. At week 14, only combined treatment sites and scaled sites showed statistically greater probing depth reduction than control sites. Conclusion: For the treatment of mild-to-moderate adult periodontitis, subgingival scaling alone is as effective as the combination of scaling and antibiotic therapy.