The harderian gland in the Wistar rat consists of tubules with wide lumina lined by a single layer of columnar epithelial cells possessing myoepithelial cells within their basal laminae. The gland contains porphyrin pigment which is stored as solid intraluminal deposits. The glandular epithelium possesses 2 cell types, termed A and B. These are characterised by an extraordinarily well-developed tubular smooth endoplasmic reticulum and numerous lipid vacuoles. Type A cells can be distinguished from type B by the number, size and content of the lipid vacuoles. Type A cells are more numerous. They contain large lipid vacuoles with dense ribbon-like material identical in form to the material in the luminal masses of porphyrin pigment, whereas those of type B cells are small with crescentic dense lamellar material. The content of the vacuoles is essentially released by exocytosis, but holocrine secretion also occurs. The lipids and the ribbon-like material represent the bulk of the intraluminal secretory product. The secretion of porphyrins seems to be associated with type A cells. The single excretory duct is lined by a stratified epithelium. The duct epithelium comprises serous cell types, designated C1 and C2 and scarce mucus-secreting cells. Type C1 cells are characterised by numerous dense granules, whereas type C2 cells are distinguished by lysosomal structures. Fibroblasts, macrophages, mast cells, plasma cells, fenestrated capillaries and unmyelinated axons are frequently observed in the connective tissue. The gland is surrounded by a collagenous capsule and an outer layer of endothelial cells of the orbital venous sinus.