OBJECTIVE: To study the expression of connexin 43 (Cx43) in various stages of oral carcinogenesis and explore the relation between Cx43 and oral mucous carcinogenesis. METHODS: 4-nitroquinoline-1-oxide (4NQO) was used for inducing oral carcinogenesis in SD rats. Tissue samples were obtained from various stages of the disease including normal oral mucosa, precancerous lesions and oral squamous cell carcinoma. Immunohistochemical method was used to determine the expression of Cx43 in various stages of oral carcinogenesis. RESULTS: In the normal rat lingual mucosa, immunohistochemical staining of Cx43 protein was mainly found in the cell membrane, weakly positive in the basal cell layer, increased in stratum spinosum and stratum granulosum, but was negative in the stratum corneum of normal epithelia. Compared with normal epithelia, was significantly decreased in dysplastic and cancerous oral epithelia the staining. The positive rates of Cx43 were respectively 100.00% (10/10), 85.71% (12/14), 66.67% (8/12), 40.00% (4/10), and 33.33% (4/12) in tongue carcinogenesis (in normal, mild, moderate and severe dysplasia, and squamous cell carcinoma tissues). The differences were statistically significant (P<0.05). CONCLUSION: Expression level of Cx43 protein was dramatically decreased with the development of rat tongue carcinoma induced by 4NQO, suggesting that abnormal expression of Cx43 protein is involved in oral mucosa carcinogenesis. Decreased Cx43 expression is an early sign of oral mucosa carcinogenesis.