The purpose of this study was to determine the effectiveness of the Water Pik oral irrigator as a vehicle for delivering an aqueous solution into periodontal pockets. Plaque-disclosing dye diluted with sterile saline solution was applied with the irrigator toward the gingival margins of teeth at 90 degrees and at 45 degrees prior to their extraction. The mean % penetration measured between a reference notch at the gingival crest and the periodontal ligament at the bottom of the pocket showed no statistical difference between the two angles of application. Penetration ranged from 44% to 71%, the lowest being into pockets 4-7 mm; higher mean penetration was noted in both subgroups 0-3 and greater than 7 mm. No statistical difference was found between proximal and facial or lingual surfaces, maxilla and mandible, existence of tooth contact, and proximal tissue contour or consistency. The mean % penetration was independent of pocket depth (chi 2 analysis). Correlation between pocket depth and mean penetration was low for all but one subgroup (90 degrees application and pockets greater than 7 mm). The results suggest that the oral irrigator will deliver an aqueous solution into periodontal pockets and will penetrate on average to approximately half the depth of the pockets.