We studied the distribution of Ca(2+)- or Mg(2+)-dependent ATPase activity in rat neurohypophysis using the lead cytochemical method of Ando et al. In electron microscopy, precipitates were found lining the outer surface of the plasma membrane surrounding nerve endings and pituicytes. These precipitates were believed to represent the activity of ecto-ATPases (as opposed to Ca pump ATPases) for the following reasons: there was equal activation by Ca2+ in the absence of Mg2+ or Mg2+ in the absence of Ca2+; the effects of the two ions were not additive; there was activation by ATP or GTP; and there was resistance to glutaraldehyde fixation, to high (10 mM) Ca2+ concentrations, and to various inhibitors such as NEM, vanadate, oligomycin, quercetin, p-chloromercuribenzoate, ouabain, and levamisole. Cytosolic activity observed in certain nerve endings in the same conditions of incubation but more sensitive to NEM is also described and discussed.