This study was designed to examine how dietary protein levels affect organ growth and protein metabolism in early and normally weaned rats. Early and normally weaned rats separated fro the dam on the 15th and 121st day postpartum, respectively. were fed diets containing three levels of protein : low(10%) , normal (20%),and high(40%) . On the 35th day, the weight and DNA, RNA and protein contents in brain , liver, and kidney were determined to ascertain organ and cellular growth. Furthermore, serum total protein , albumin , $alpha$-amino N and creatine and urinary urea N, and creatinine were determined in order to ascertain protein metabolism and renal functions. Dietary protein levels were not observed to significantly affect total DNA content, which may represent an index of cell number in the liver, brain and kidney. Fresh weight and protein/DNA ratio, which may represent indices of cell size, significantly increased in proportion to dietary protein in the kidney. As for the early weaned rats , the liver cell size significantly decreased. Dietary protein levels and weaning periods did not affect serum total protein and albumin . However, serum urea-N significantly increased in proportion to dietary protein levels whereas serum $alpha$-amino N was decreased by early weaning . Nitrogen retention was lower in early weaned rats fed low or high levels of protein than in normally weaned rats. The results demonstrate that low or high levels of dietary protein have less desirable effects on protein metabolism in prematurely weaned rats.