The ultrastructure of the principal cells and intraepithelial leucocytes in the initial segment of the rat caput epididymidis was examined with the electron microscope. Specializations of the principal cells associated with absorption include numerous endocytic invaginations of the cell surface, numerous coated vesicles and multivesicular bodies in the apical cytoplasm. It was demonstrated that particulate tracers are taken into the cells and sequestered in secondary lysosomes and multivesicular bodies. Morphological features consistent with secretory activity are also found in the principal cells and include numerous cisternae of rough endoplasmic reticulum with a flocculent grey content and an extremely well-developed Golgi apparatus. The speculation that the principal cells are actively secretory despite the absence of secretory granules formed in the Golgi and of a visible mechanism for release of the product at the cell surface is discussed.The "halo cells" in the epididymal epithelium were also examined and it is shown that many of these cells are not typical migratory lymphocytes, Chief among the differences are their granule-containing multivesicular bodies and more abundant endoplasmic reticulum. Nonetheless, it is conceivable that the halo cells are lymphocytes and that the conditions they encounter as they leave the circulation and enter the epididymal epithelium may stimulate morphological changes. The possible immunological significance of these observations is discussed.There have been several descriptions of the ultrastructure of the epididymal epithelium in recent years (Nicander, '65; Friend and Farquhar, '67; Flickinger, '69; Hamilton, '72a,b). The improved preservation and higher resolution achieved in these studies have corrected a number of persistent misconceptions of early light microscopists who worked on the epididymis. Thus, irregular surface protrusions previously interpreted as a manifestation of apocrine secretion have been shown to be artifacts of fixation and certain granules formerly regarded as secretory products have been shown to be lysosomes. The structural differentiation of the cytoplasm has proven to be far more elaborate than previously supposed and it is quite evident that this eoithelium is by no means the metabolically inactive lining of an organ for sperm storage. The functional corre-ANAT. REC., 175: 169-202.lates of the cytological features are largely unknown, and, although considerable evidence is available on the morphology of the absorptive process in principal cells (Friend and Farquhar, '67; Nicander, '65; Burgos, '64), the morphology of secretion of glycerophosphorylcholine (Dawson, Mann and White, '57; Dawson and Rowlands, '59), carnitine (Marquis and Fritz, '65), sialic acid (Rajalakshmi and Prasad, '69) or steroids (Hamilton et al., '69; Hamilton, '72b) has not been elucidated.In the present paper we report the mor-
