The present investigation compared subgingival metronidazole in dialysis tubing and subgingival chlorhexidine irrigation in the control of chronic inflammatory periodontal disease. 10 patients with 4 mm or deeper periodontal pockets were divided into 2 groups. Both received baseline scaling, root planing, subcontact area cleaning and instruction in the Bass technique of tooth brushing, but not in interdental cleaning. One group with 80 pockets received 0.2% chlorhexidine subgingival irrigation for 28 days and the other with 86 pockets received 0.5% metronidazole solution incorporated in subgingivally placed dialysis tubing. The tubings were replaced with freshly filled ones at days 7, 14 and 21. Active treatment ceased at day 28. Plaque Index, Sulcus Bleeding Index, pocket depth and gingival shrinkage were recorded at days 0, 7, 14, 21, 28, 56 and 84. Subgingival 0.5% metronidazole in dialysis tubing and 0.2% chlorhexidine irrigation were found to be equally effective in reducing chronic periodontitis. Metronidazole reduced Plaque Index less but pocket depth more than chlorhexidine. Improvements were maintained significantly below baseline levels for at least 8 weeks after the end of the 4-week treatment period.