The distribution of elastic fibres in the human ductus deferens from birth to senility was studied by light and electron microscopy. Elastic fibres are lacking in the ductus deferens in infants and children. In the adult ductus deferens, they form two layers in the lamina propria: (1) an inner layer of circumferentially oriented elastic fibres, and (2) an outer meshwork of elastic fibres. Elastic fibres are also present in the narrow intercellular spaces between the smooth muscle cells of the muscular coat, mainly in the inner muscular layer. A layer of elastic fibres surrounds the muscular coat. The ductus deferens of ageing subjects shows fragmentation and disorganisation of the elastic fibre layers of the lamina propria. Elastic fibres in the muscular coat are more abundant than in younger adults, forming larger bundles. Electron microscopy demonstrated the presence of immature elastic fibres at puberty as collections of microfibrils, some of them containing loci of amorphous substance (elastin). In the adult ductus most elastic fibres have a mature appearance. The amount of amorphous substance has increased and the number of microfibrils has decreased. Electron-dense inclusions are present within the amorphous substance. With advancing age the amorphous substance forms large, structureless masses showing abundant electron-dense inclusions and areas of rarefaction. A thin layer of microfibrils is present only at the periphery of the elastic fibres. Whether or not androgenic hormones are in any way involved in the formation of elastic fibres in the ductus deferens and testis is something which requires further study.