Protein utilization of the diet was tested in relation to different levels of magnesium and zinc in rats. The experimental diets contained either a low (0.14 g Mg or 10 mg Zn/kg) or adequate (0.45 g Mg or 40 mg Zn/kg) level of Mg or Zn and two different quality protein sources: casein or wheat gluten. Net protein utilization and net protein radio indexes in case of casein were significantly lower for the diet containing a low level of Mg or Zn. For gluten diets, such differences were not observed. Digestibility of protein measured in rats fed a low Mg or Zn casein diet was the same as for the diets with an adequate content of these minerals. Rats fed low Mg or Zn casein diets showed a significantly lower plasma Mg or Zn and a lower liver DNA content in comparison to the rats on adequate Mg or Zn diets. The results indicate that the utilization of protein is affected by Mg and Zn content of the diet and that this relationship depends on the quality of protein.