The effects of dietary arginine deprivation during gestation and lactation were studied in the rat. Placental development, as indicated by the total protein and RNA content, of rats fed a diet devoid of arginine was inferior to that of pair-fed controls. Fetal cerebral protein plus body weights were reduced by maternal arginine deprivation. Urinary excretion of orotic acid and citric acid was significantly increased in arginine-deficient rats throughout the gestational period. Pups nursed by arginine-deficient dams during lactational periods had smaller body and organ weights than those nursed by pair-fed control dams regardless of their prenatal nutritional states. Nursing performance of the dam was most affected by feeding arginine-free diet throughout gestation and lactation. Consumption of an arginine-free diet by the dam during gestation resulted in lower birth weight and higher perinatal mortality of the pups nursed. Deletion of arginine from the diet after parturition significantly reduced nursing performance after 18 days of lactation. The reduction in nursing performance caused by arginine deficiency during gestation could be corrected by replenishing arginine after parturition. Mammary gland development, as indicated by nucleic acid content, was reduced by either reducing dietary arginine or food intake during lactation. All these results provided evidence that dietary arginine is required during gestation and lactation for optimal reproductive response and nursing performance in rats.