The purpose of this study was to evaluate clinically and histopathologically the effects to the periodontal tissue in rats after Cyclosporin A(CsA) administration and to determine whether there is a relationship between dosage of CsA or blood CsA level and the severity of gingival overgrowth in rats. Twenty 6-week-old Sprauge-Dawley rats were randomized into 4groups. The control group received olive oil only and the test groups received daily CsA in olive oil via gastric feeding for 6weeks at a 3,10, and 30 mg/Kg. Rats were weighed to evaluate the systemic effect of drug and stone models were made from alginate impressions of upper and lower anterior region at 2-week interval. On completion of oral CsA administration, blood were collected and blood CsA levels were quantitated by TDxFLx analyzer. Rats were sacrificed and their upper and lower jaws were removed together with the surrounding gingiva and soft tissue for light microscopic examination. The results were as follows: 1. The weight gain of CsA-treated rats was much less than of the control group and central incisors were gradually displaced and separated in the test groups. 2. The extensive fibrovascular proliferation and scattered inflammatoy infiltrates in an edematous stroma were observed in enlarged gingiva of CsA-treated rats. 3. The increase in buccolingual, mesiodistal dimension of the anterior teeth and vertical height of the interdental papilla showed dose-dependent manner in CsA-treated rats. 4. Significant positive correlation exists between blood CsA level and the severity of gingival overgrowth in anterior teeth. This result indicates that the severity of gingival enlargement in CsA treated rats is correlated with dosage of CsA administration and blood CsA level.