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Female rats fed practical diets containing 18 or 25% protein produced and weaned a few more young than those fed 30 or 35%. Females fed purified diets containing 14, 17 and 20% soybean protein in different periods of the reproductive cycle produced as many young as the control group which consumed 25% soybean protein. With 10% protein throughout the cycle, the number of young and weaning weight were reduced to less than those of the controls. With 10% protein during mating and gestation period, the number weaned and their weight was essentially equal to that of the control group. With 14% protein during 2 or more periods, the weight was less than that of the young in the control group. The number of young weaned by females fed a practical diet containing 3 to 18% fat was essentially the same but the number weaned was less with 25% fat. With a purified diet reproduction by females consuming 3 or 20% fat was superior to those fed 36 and 50%. When the protein-metabolizable energy was adjusted to a constant ratio (0.074 to 0.070) the effect of a high fat diet on the number of young weaned and the weaning weight was very small.