Oral carcinogenesis is a multistep process and requires accumulation and interplay of a series of molecular genetic events. Gap junctions are intercellular channels composed of connexin subunits that mediate cell-cell communication. The disfunctions of gap junctions are believed to be associated with cancer development. We therefore investigated the expression of connexin (Cx)43, one of the major connexins in oral epithelia, during 4-nitroquinoline-1-oxide-induced rat tongue carcinogenesis. By immunohistochemistry, Cx43 expression was observed mainly in the cell membrane in normal rat oral epithelia. It was weak in the basal cell layer, increased in the stratum spinosum and stratum granulosum, and negative in the stratum corneum of normal epithelia. Throughout the course of carcinogenesis, both Cx43 immunostained area and mean intensity decreased with significant difference among various histopathological groups (P < 0.05). In cancerous oral epithelia cytoplasmic staining could be observed. However, Cx43 mRNA level showed no significant difference in the progress of oral carcinogenesis (P > 0.05) and without correlation to Cx43 protein immunostained area and mean intensity. Our results indicated that downregulation of Cx43 might be an early event during oral carcinogenesis, which could be a biomarker for early changes in oral malignant transformation.