The testicular and prostatic ends of the vasa deferentia of two male monkeys weighing 3-4 kg were examined ultrastructurally, using the method described by Richards & Tranzer (1975). The general ultrastructure of the vas deferens resembles that described previously. In addition, new features were observed. These include (i) shedding of parts of the apical portions of the epithelial cells; (ii) darkening of mitochondria-rich cells in the epithelium; (iii) presence of secretory granule-like structures in the principal cells of the epithelium; (iv) mast cells which may be associated with blood vessels or smooth muscle cells, and (v) presence of dense osmiophilic cells in the lamina propria and muscle coat, but predominantly in the former. The last-mentioned cells are stellate, having many branching processes, and are characterised by the appearance of many caveolae on their surface membranes. Basal lamina may or may not be seen to surround the cell membranes. The cells are contacted by nerve varicosities which resemble those of cholinergic and purinergic fibres.