Cyclosporin-A (CsA) which is an effective immunosuppressive agent to control the rejection of organ transplants causes gingival overgrowth as one of accompanying side effects. However, the mechanism of this particular gingival overgrowth still remains unclear. This seems to be largely due to the difficulty of reproducing gingival overgrowth in experimental animals, especially in rodents. The purpose of the present study was to establish a rat model for CsA-induced gingival overgrowth. Specific pathogen-free Fischer and Sprague-Dawley rats (20 days old) were used and fed a caries inducing diet containing CsA. The degree of gingival overgrowth was measured, and histological examination of the gingival sections was performed. The results obtained were as follows; 1. Marked gingival overgrowth was noted around mandibular molars of all rats fed died 2000 containing 56% sucrose supplemented with CsA (a diet of 120-200 mg/kg). This overgrowth was more conspicuous in buccal than in lingual gingiva. However, there were no differences between the two strains of rat in the macroscopic findings as well as in the severity of CsA-induced gingival overgrowth. 2. Rats receiving CsA treatment for 40 days (at 63 days of age) showed the maximum level of gingival overgrowth in both strains of rats. When the gingival sulcus depths at 48 days of age were compared, the S.D. rats seemed to be a higher responder to CsA than the Fischer rats. 3. CsA treated rats showed significantly higher plaque scores than untreated rats. However, CsA-induced gingival overgrowth was little affected by this enhancement of plaque accumulation in the rats infected with S. sobrinus.(ABSTRACT TRUNCATED AT 250 WORDS)