Four- or 12-week-old rats were fed diets containing either 25 or 45% casein and either 0.35 or 0.8% phosphorus (P) for 7 weeks. Calcium (Ca) level of each diet was 0.9%. In the young, fast-growing rat (expt 1), plasma P was increased (P less than 0.01) when 0.8%, rather than 0.35%, P was fed. In the more mature rats (expt 2), increased dietary P depressed Ca in plasma but elevated P; high protein intake elevated whole-blood Ca, but depressed plasma P. Dietary treatment had little effect on muscle of mature rats, but wet weight and total P content of muscle were higher in young rats fed 0.8%, rather than 0.35%, P. Femur dry weight and P concentration were greater when 0.8%, rather than 0.35%, P was fed to young rats; dietary P did not affect any other parameter of bone (expt 1, 2). Excessive protein intake elevated femur P concentration in young rats, but depressed length and strength of femurs in mature rats. Some parameters of bone, blood and muscle can be influenced by dietary levels of protein and P. The response, however, is dependent on the physical maturity of the rat when dietary treatment begins.