Carbonic anhydrase (CA) activity was histochemically localized in the elasmobranch rectal gland at the light and electron microscopic levels. Reaction product in the secretory tubules was localized coincident with that reported for sodium-potassium activated adenosine triphosphatase (Na-K-ATPase): along the highly amplified basolateral plasma membranes of the epithelial cells. Reaction product was also localized along the plasma membrane of adjacent central canal epithelial cells. The results suggest that CA plays a role in modulating the environment of the intercellular space which in the secretory tubule is believed to be the paracellular pathway for sodium. The results also draw attention to the possible role of the central canal epithelium in modification of the secreted fluid.