The effect of dietary Zn or Cu deficiency on the primary free radical defense system was examined in the lungs and livers of 6-wk-old rats. Enzymatic components (superoxide dismutase, catalase and glutathione peroxidase) and nonenzymatic components (alpha-tocopherol, ascorbate, glutathione and metallothionein) of the primary free radical defense system, as well as tissue concentrations of Cu, Zn and Fe, were measured. Liver CuZn-superoxide dismutase and liver catalase activities were significantly lower (P less than 0.05), and lung metallothionein and liver ascorbate concentrations were significantly higher (P less than 0.05) in Cu-deficient rats than in either pair-fed or ad libitum-fed controls. Zn-deficient rats had a significantly lower (P less than 0.05) concentration of liver metallothionein than either control group. Other changes in the enzymatic and nonenzymatic components of the primary free radical defense system could be attributed to the reduction in feed intake by Zn- or Cu-deficient rats and not to a direct effect of the Zn or Cu deficiency per se. The primary free radical defense system in lung and liver of severely Zn- or Cu-deficient rats was not seriously compromised.