Indwelling catheters were used to collect fluid directly from the medial and lateral segments of duck nasal salt glands showing, for the first time, that the secretions are fully hypertonic before reaching the medial and lateral drainage ducts. Using this method it was possible to show that (a) there is a functional symmetry between the left and right salt glands, (b) the medial segment always secretes fluid at approximately twice the rate of the lateral segment and (c) fluid secreted by the medial segment has the same ionic composition but variable ion concentrations when compared with fluid from the lateral segment. A 12 % increase in post-segmental fluid osmolality was probably due to the evaporation of water from epithelial surfaces in the nasal cavities during breathing. A post-segmental outflux of Ca²⁺, Mg²⁺ and Cl^- in the medial and lateral collecting ducts and/or nasal epithelium may be of adaptive significance when birds inhabit calcium and magnesium rich marine environments.