After mating, rats were fed one of the following diets: 100% sucrose, 100% vitamin-free casein, purified protein-free or purified with 20% casein. Fetal survival and weight changes in the maternal adrenal glands, uterus, ovaries and gastrocnemius muscle were determined at Day 20. Pregnancy failed in all animals fed sucrose or a protein-free diet and in half of those fed purified casein. Daily injections of progesterone (5 mg) increased both the number of rats remaining pregnant and fetal survival rates in those fed sucrose, a protein-free diet or casein. Pregnancy maintenance in these animals was characterized by larger adrenal glands and smaller gastrocnemius weights. The failure of pregnancy in rats fed only vitamin-free casein or a protein-free diet was associated with a significant increase in loss of total body weight. The minimum period of progesterone treatment for the maintenance of pregnancy was Days 5 to 9 in dams fed only sucrose after mating. In animals fed sucrose, treated with progesterone from Days 5 to 9 and killed on Days 6, 9, 12, 15 and 18, the weights of the uterus and conceptuses began to increase, the weight of the liver declined, followed by a decrease in gastrocnemius weight. Exogenous ACTH or prolactin was inadequate for maintaining viable embryos in ovariectomized rats given a diet of sucrose.