Changes in diet quality may be as important as changes in energy availability in the reproductive success of small mammals. The effect of dietary protein on fitness of females is most likely mediated by changes in yield or composition of milk. Mus musculus was used as a model to test whether dietary protein restriction leads to changes in milk composition. Primiparous mice were fed a 10% or 20% casein-based diet through 2 consecutive reproductive attempts. There was no effect of diet on food consumption or litter size. However, a reduction in dietary protein led to the production of milk with a 12% decrease in protein content and decreased both prenatal and postnatal growth of young. The impact of low levels of dietary protein on milk protein was greater with the increased reproductive demands of concurrent pregnancy and lactation. Significant variation in the protein content of milk was seen among females on the 20% casein-based diet, but not among females on the 10% casein-based diet. Further work is needed to determine the physiological mechanisms that lactating mice might possess to compensate for changes in dietary protein levels, and to determine if the effect of decreased diet quality on the reproductive success of small mammals is similar to the effect of energy availability.