The Harderian gland, its secretory duct and porphyrin content were examined in the mongolian gerbil (Meriones unguiculatus). The gland consisted of tubules lined by a single layer of epithelial cells and a myoepithelial network. The tubule cells were often binucleate and possessed lipid vacuoles in the apical half of the cell, a corona of granular endoplasmic reticulum surrounding the nucleus, and cytoplasmic 'slashes'. The latter are probably derived from dense membranous couplets and may be precursors of the lipid vacuoles. Holocrine and merocrine secretion was observed. Interstitial cells included plasma cells, mast cells and (predominantly) melanocytes which render the gland black. The gland was surrounded by a collagen capsule and an outer layer of highly attenuated (possibly endothelioid) cells. Within the gland, the secretory duct was lined by a single layer of normal tubule cells. Outside the gland, the duct enlarged to form an ampulla, from which clefts led off to deep crypts. The ampulla and clefts were lined by cells with small dense apical granules and stubby microvilli; some possessed lipid vacuoles. The crypts were lined by serous cells with active Golgi regions. At the duct opening, ampullary cells became squamous and goblet cells occurred. Geometric crystalloid deposits (with a layered structure of 7.6 nm periodicity) occurred at cleft-crypt junctions. Islets of extra-glandular ductal tissue were occasionally found within the gland. Porphyrins were detectable both by chemical assay and fluorescence microscopy. There was a trend for female glands to have a higher content than males. Solid intraluminal accretions of porphyrin and/or lipid were present.