The bovine rumen epithelium is a major site for absorption of rumen microbial fermentation products (e.g. volatile fatty acids) (Stevens and Settker, 1966, Steven and Marshall, 1970) but may also contribute to maintenance of an optimal milieu for rumen micro-organisms. Here we define a transport model for this epithelium by using electron and light microscopy combined with immunocytochemistry. Bovine rumen samples were obtained from a commercial abattoir. After SDS-PAGE separation of proteins from bovine rumen epithelial homogenates, Western-blotting demonstrated the expression of claudin 1 (an integral plasma membrane tight-junctional protein), connexin-43, (gap-junction monomer), and the catalytic alpha subunit of the Na+-K+ ATPase. After methanol-fixation and cryo-sectioning, four cell layers could be distinguished in the epithelium, namely the stratum corneum, the stratum ganulosum, the stratum spinosum and the stratum basale. Claudin 1-immunostaining showed a peripheral cellular staining consistent with plasma membrane and was most intense at the cells of the stratum granulosum. Staining intensity decreased through the stratum spinosum to the stratum basale which showed only weak staining. The stratum basale was negative. Morphological identification of occluding tight junctions at the outermost layer of the stratum granulosum was confirmed by electron microscopy Connexin-43-immunostaining was most intense at the stratum granulosum and stratum spinosum decreasing towards the stratum basale, staining was consistent with plasma-membrane staining. There was intense immunocytochemical staining of the stratum basale for the Na+-K+ ATPase, with weak staining of the stratum spinsosum. Both the stratum granulosum and the stratum corneum were devoid of apparent staining. Foci of intense staining of the mitochondria-rich stratum basale cells in the region of the basement membrane were associated with extensive infoldings and interdigitations of the basement membrane and lateral space between stratum basale cells respectively. Thus polarity of Na+-K+ pump distrubution within the stratum basale cell reflects membrane amplification. We conclude that epithelial barrier function may be attributed to the stratum granulosum, whilst cell-cell gap junctions allow diffusion to interconnect the barrier cell layer with the stratum basale where Na-K pump density is concentrated.