Extract: The effect of zinc deficiency on the brain of the suckling rat was studied by feeding a zinc-deficient (Zn-) diet to dams from parturition and measuring the in vivo uptake of 3H-thymidine by brain DNA and the 35S uptake by brain protein as well as the brain lipid composition of their pups. These results were compared with similar data obtained on pups nursed by pair-fed (PF) and ad libitum-fed control dams which had been given zinc in addition to the diet.The specific activities (SA) of DNA and protein were decreased in the brains and livers of the pups nursed by the zinc-deficient dams compared with pups nursed by dams pair-fed with the zinc-deficient dams and hence semistarved (SA of brain DNA: Zn- = 5.90 versus PF = 11.36; SA of liver DNA: Zn- = 5.16 versus PF = 15.8; SA of brain protein: Zn- = 50.20 versus PF = 85.30; SA of liver protein: Zn- = 56.02 versus PF = 79.20). On the other hand, similar concentrations of DNA, RNA, and protein were present in their brains as well as their livers. The concentrations of total lipid were decreased in the brains of the deficient pups compared with the controls (Zn- = 0.215 versus PF = 0.309 mg/mg of brain), while the concentrations of brain phospholipid and the brain fatty acid patterns were similar.Zinc deficiency has thus been shown to impair DNA and protein synthesis in brain of the suckling rat as it has previously been shown to do in other tissues including liver and epiphyseal plate.Speculation: Zinc is essential for synthesis of nucleic acids and protein. In individuals with protiencalorie malnutrition, an associated zinc deficiency may compound the effects of protein deprivation by impairing utilization of the limited protein available. Zinc deficiency in the human may thus contribute to the impaired brain growth reported to occur in infants with protein-calorie malnutrition.