To demonstrate the effect of zinc and/or calcium deficiency on calmodulin concentration and also the antagonistic behavior of zinc and calcium in metabolism, four groups of eight rats each were provided the following diets in equal amounts: group I, the basic diet with 63 ppm zinc and 0.88% calcium; group II, the calcium-deficient diet (0.0043% calcium); group III, the zinc-deficient diet (1.1 ppm zinc); and group IV, the combined zinc/calcium-deficient diet. The zinc-deficient diet significantly reduced the zinc concentration in serum (72%), femur (65%), pancrease (34%), muscle (23%), testis (11%) and brain (7%), and elevated the calcium concentration in testis (19%) and brain (24%), but did not affect calcium concentration in serum, pancreas and muscle. Likewise, calcium deficiency reduced the calcium concentration in serum (56%), femur (40%), pancreas (16%), muscle (43%), testis (19%) and brain (24%), and elevated the zinc concentration in femur (22%), although the zinc concentration in serum, pancreas, muscle and testis remained unaffected. Combined zinc and calcium deficiency elevated both the zinc as well as the calcium concentration in serum, femur and muscle, as compared to zinc or calcium deficiency alone. The alkaline phosphatase activity was reduced by 52% during zinc deficiency and was elevated by a factor of 2.5 during calcium deficiency. The calmodulin concentrations in muscle, testis and brain, as determined by radioimmunoassay, were elevated during both zinc as well as calcium deficiency, although this elevation could not be confirmed in every case.