A strain of mice with a well-defined but short life-span of about 1 year was given a solution of prednisolone phosphate (2·1) νg.ml) as the only source of drinking water from weaning. This treatment retarded growth by about 25% and increased the life span to about 2 years. Several biochemical features of development (the increase in tissue collagen, the fall in the respiratory capacity of liver, the rise in the calcium to collagen ratio of bone and the decrease in kidney ATPase) did not proceed as fast as normal in prednisolone treated animals. © 1968.