The three-dimensional microvasculature of the nasal salt gland of the duckling was studied by vascular corrosion casting and scanning electron microscopy. Changes in the vascular volume of the gland in response to osmotic stress were also determined using cast weights and densities. The richly vascularized gland is supplied on its medial surface by large branches of the supraorbital and ethmoidal arteries. Numerous arterial branches enter the gland and distribute to lobes via the interlobar connective tissue. Lobar arterioles penetrate to the periductal areas of the lobes before dividing into capillaries supplying the ductal epithelium and secretory tubules. Capillaries envelope the secretory tubules and run radially from the ducts toward the lobe periphery, so that blood flows counter to the tubular secretion. Blood is collected via venous plexuses seen as distinct drainage units on the periphery of each lobe. Veins exhibit large numbers of bicuspid valves. Following 1 day and 4 days of osmotic loading (feeding 1% NaCl), vascular volume of the gland increased fivefold and ninefold, respectively, a response that precedes and exceeds that of the gland weight or Na,K-ATPase activity. When salt water-adapted ducklings were fed fresh water for only 24 hr (deadaptation), vascular volume fell to 2.8 times the control level. Changes in blood flow to the gland during osmotic adaptation and deadaptation are rapid and dramatic and may represent the initial steps in the control of gland secretion.