In fish of all groups examined including Teleostei, Chondrostei, Holostei, Chondrichytes, and Dipnoi, the primary epithelium that surrounds the primary lamellae has a close relationship with the venous compartment. Except in Dipnoi that displays a specialized epithelial drainage, the venous compartment consists of a central venous sinus that is connected with the systemic vasculature by noninnervated muscular arteriovenous anastomoses and drains into the branchial veins. Primary epithelium contains the chloride cells, which vary in morphology and number according to the milieu where the fish lives. The presence of an accessory cell beside the chloride cell is characteristic of seawater or seawater-adapted fish. The secondary epithelium that covers the free part of the secondary lamellae has an exclusive relationship with the arterioarterial vasculature, i.e., the pillar capillary compartment. This compartment is actively controlled by innervated sphincters located in the primary lamellae and in lower species by pre- and postlamellar noninnervated sphincters. Contraction of pillar cells may also contribute to this control. The secondary epithelium consists of an outermost layer of pavement cells that exhibited structural characteristics suggestive of cell coat secretion and an innermost layer of less differentiated cells. In contrast to the primary epithelium, the secondary epithelium does not exhibit any obvious differences between freshwater and seawater fish or undergo any obvious change during transfer of fish from fresh- to seawater. However, in conditions which exaggerate the absorptive functions of freshwater chloride cells, the secondary epithelium become modified by an intensive differentiation of freshwater chloride cells from its innermost layer of cells. These observations suggest possible specialized functional relationships between seawater chloride cells and the central venous sinus, and freshwater chloride cells and the arterioarterial compartment.