Infection is detrimental to growth, making it a serious problem for both humans and animals. To explore factors responsible for the restoration of tissue protein stores following infection, we developed a model of Escherichia coli infection suitable for studies of protein metabolism in growing animals. Weanling female Sprague-Dawley rats were given a single intraperitoneal injection of E coli (approximately 10(9) colony forming units [CFU]/kg body weight); control rats were injected with vehicle alone. Two days later, infected rats had lower (P less than .05) body weight (-11%), food intake (-49%), daily nitrogen balance (-64%), and lower protein mass of gastrocnemius muscle (-21%), tibialis anterior muscle (-23%), gastrointestinal tract (-15%), skin (-16%), and carcass (14%), compared with control rats. There was complete catch-up in terms of body weight, cumulative nitrogen balance, and carcass and tissue protein masses within 3 weeks of infection. Catch-up was achieved predominantly by increasing (P less than .05) the apparent nitrogen digestibility of the diet, thereby increasing the amount of nitrogen available to the body. Food intake did not increase and urinary nitrogen did not decrease during catch-up growth. E coli peritonitis in weanling rats provides a simple and useful model for the study of the effects of infection on growth and protein metabolism in growing animals.