The rectal gland of the spiny dogfish, Squalus acanthias has been described by light and electron microscopy. The cell outline shows remarkable complexity in electron micrographs. The basal plasma membrane has numerous complex infoIdings and interdigitations separated by groups of mitochondria. The lateral plasma membrane also shows extensive interdigitation. The apical region demonstrates profiles of interleafing cell processes with much membrane interdigitation occurring between them but containing few mitochondria. The apical surface has microvilli which extend into the tubular lumen and are covered with nap-like filaments. Glands of some fish were catheterized and attempts were made to correlate the fine structure of these glands with the known secretory rates. The cellular morphology of the secretory cells of the rectal gland correlates well with that seen in other ion-transporting tissues, although some unique specializations were noted.Ion transport pervades all of biology. In spite of efforts by scientists of many fields, the basic mechanisms of ion accumulation and pumping remain unknown. One approach to the problem of salt secretion is to examine the structure of cells which are highly specialized for this activity. The rectal gland of the spiny dogfish, Squalus acanthias, is such a tissue. Burger and Hess ('60a, b ) catheterized the duct of the gland and have shown that this gland was capable of secreting a sodium chloride solution with a concentration about twice that of the blood plasma and greater than that of sea water. They also showed that the volume of fluid excreted was large enough to eliminate significant amounts of sodium chloride from the fish, this presumabIy being its function. Burger ('62) also demonstrated that the gland secreted at a greater rate after the fish received a large dose of salt solution. He could find no evidence of neural control.Important articles from the older literature on the anatomy and physiology of elasmobranch rectal glands were those of Pilliet (1885), Crawford (1899), Hoskins ('17), and Crofts ('25). The recently published physiological studies proposing a salt-excreting function have awakened a new interest in the morphology of this gland. Abstracts have been published on the histochemistry of elasmobranch rectal glands by Bernard and Hartmann ('60) on the histology and ultrastructure of rectal glands from Squalus acanthias by Bulger ('61) and on the ultrastructure of the glands of Urolophus (round sting-ray) by Doyle ('62a). IR addition, Doyle ('6%) recently published an article on the fine structure of the secretory cells of Urolophus with comments on the structure seen in Squalus acanthias.Various structures have been associated with the function of salt secretion, but no unique structure has been identified with certainty as being responsible for salt concentration. Rather, it appears that the rectal gland, in general, resembles the structure, both coarse and fine, of other electrolyte-separating organs. This paper substantiates such a concl...