Zinc deficiency was induced in two series of weanling rats fed a zinclow diet (0.5 mg/kg) and a zinc-higher diet (1.4 mg/kg). Symptoms of zinc deficiency included loss of hair, severe growth retardation and scaly paws. Parakeratotic lesions were observed in cheek, soft palate and esophagus. A reduction in kidney and intestinal alkaline phosphatase activity was observed in both groups. The addition of 1 x 10^-5 M zinc as Zn acetate to the kidney and intestinal homogenates assayed at pH 10.5 increased the activity of alkaline phosphatase in zinc-deficient animals and their pair-fed controls, but significantly more so in zinc-deficient animals. In the presence of zinc, intestinal enzyme activity reached similar levels in all groups of animals. Magnesium in the same concentrations failed to increase the enzyme activity, instead caused a slight but consistent inhibition. It is concluded that in zinc deficient animals the cells keep on synthesizing apo alkaline phosphatase, which can be activated by the addition of zinc to the tissue homogenates from zinc-deficient animals.