In situ release kinetics of 25% tetracycline-loaded fibers (Actisite) were evaluated in this study. 1 cm precut tetracycline fibers were removed from periodontal pockets (>5 mm) of 24 patients. Fibers were maintained at their site for 1, 3, 7, 10 and 12 days. Remaining biologically active tetracycline on the fibers was determined by agar diffusion inhibition assay. Inhibition zones induced by fibers on P. gingivalis (256) seeded BHI agar were measured after 48 hours of anaerobic incubation. Data were evaluated by one-way analysis of variance with Post-hoc Duncan analysis. Results of this study show there is a considerable decrease in the amount of tetracycline remaining in the fibers removed between 1-3 and 10-12 days. The amount of active drug remaining in the fibers at days 3 and 7 was comparable and indicates no significant deposition between these two days.