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To test the hypothesis that reduced protein synthesis may increase life span by retarding genetic informational transfer during early life and reducing the use of the genetic code and thereby minimizing genetic imperfections as they may occur during late life, two approaches were used. In the first protein synthesis was depressed by the administration of cycloheximide, in the second by reducing the dietary protein level. One-day-old chick embryos were injected with either 0.8 gamma or 1.0 gamma of cycloheximide. On the second and third day of incubation both stage of development and heart rate were lower in the treated embryos. Growth was retarded throughout the 17 days of incubation as measured by size and DNA contents. As estimated by the activities of various enzymes per unit DNA, cells of the treated embryos were the same as normal ones of the same age. Sixteen-month-old female Wistar rats which had been previously maintained on a commercial diet (23.4% protein) were fed diets which contained either 24, 12, 8 or 4% casein throughout their remaining life span. Except for a lowering of the body weights of the animals fed the 4% casein diet, the body weights of the remaining animals were unchanged. Reducing the dietary protein level from 24% to 12% increased the life span (25%) of the animals.