Fully crossed, factorially arranged experiments showed that, under defined conditions, interactions occur between nickel and iron, nickel and copper, arsenic and zinc, and possibly vanadium and chromium. Nickel and iron interacted when dietary iron was supplemented as ferric sulfate only. Signs of nickel deprivation were more severe when dietary iron was low; or the signs of moderate iron deficiency were more severe when dietary nickel was deficient. When iron was supplemented to the diet as a 60% ferric-40% ferrous sulfate mixture, nickel and iron apparently did not interact. The findings suggested a synergistic relationship between nickel and iron when dietary iron was in a relatively unavailable form. An antagonistic interaction between nickel and copper was found when dietary iron was supplemented as a 60% ferric-40% ferrous sulfate mixture. Signs of copper deficiency were more severe in nickel-supplemented than in nickel-deprived rats. When the rats were made severely iron deficient by feeding of low levels of ferric sulfate only, no apparent interaction between nickel and copper was found. The interaction between arsenic and zinc apparently was noncompetitive. When dietary zinc was 40 microgram/g, arsenic-deprived chicks exhibited depressed growth and elevated hematocrits. In zinc deficiency, growth was more markedly depressed and hematocrits more markedly elevated in arsenic-supplemented than in arsenic-deficient chicks. Arsenic might be necessary for the efficient utilization or metabolism of zinc. Findings indicating an interaction between vanadium and chromium were tentative. In one experiment, the addition of 500 microgram of chromium/g of diet apparently made 5 micrograms of vanadium/g of diet toxic for chicks. Thus, the interactions between essential trace and ultratrace elements might be of nutritional significance.