The monoptychic 'apocrine' scent glands of the sternal region of two adult male Tupaia belangeri were studied by transmission and scanning electron microscopy, in order to assess the modes of release of their secretory products. In segments of the secretory tubules with a narrow lumen the epithelial cells are columnar and are firmly connected to each other by desmosomes and junctional complexes. Myoepithelial cells are interspersed between the secretory epithelium and the basement membrane. The cytoplasm of the secretory epithelial cells contains granular and agranular endoplasmic reticulum, mitochondria, Golgi apparatus, lysosomes and secretory granules. The free surface of the secretory epithelium is furnished with densely arranged microvilli. The apices of the cells protrude as dome-shaped extensions into the glandular lumen. At the tip of these extensions cellular processes of irregular shape are found. The surface of these processes shows no microvilli. They contain densely packed and dilated cisternae of agranular endoplasmic reticulum and free ribosomes. The constriction of the base of these processes and the occasional observation of a 'demarcation membrane' between a process and the secretory cell indicate that these secretory processes become detached from the secretory cells according to the apocrine mode of extrusion. Within the glandular lumen they break down and form secretion. Parallel to this apocrine extrusion the same cells produce secretory granules, the diameter of which measures 300-900 nm. The contents of these granules are released into the glandular lumen by exocytosis according to the eccrine mode of secretion. This resembles the situation in other mammals in which monoptychic glands release their secretory products by means of both apocrine and eccrine extrusion. Therefore monoptychic skin glands should not be classified into apocrine and eccrine glands. Our results corroborate Schaffer's (1927, 1940) more general classification of exocrine glands according to the nature of the secretory epithelium into monoptychic and polyptychic glands.