The distribution of Na § pump sites (Na+-K+-ATPase) in the secretory epithelium of the avian salt gland was demonstrated by freeze-dry autoradiographic analysis of [3H]ouabain binding sites. Kinetic studies indicated that near saturation of tissue binding sites occurred when slices of salt glands from salt-stressed ducks were exposed to 2.2 ~M ouabain (containing 5 tzCi/ml [ZH]ouabain) for 90 min. Washing with label-free Ringer's solution for 90 min extracted only 10 % of the inhibitor, an amount which corresponded to ouabain present in the tissue spaces labeled by [14C]inulin. Increasing the KC1 concentration of the incubation medium reduced the rate of ouabain binding but not the maximal amount bound. In contrast to the low level of ouabain binding to salt glands of ducks maintained on a freshwater regimen, exposure to a salt water diet led to a more than threefold increase in binding within 9-11 days. This increase paralleled the similar increment in Na+-K+-ATPase activity described previously. [3H]ouabain binding sites were localized autoradiographically to the folded basolateral plasma membrane
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