The relationship of dietary protein to cadmium absorption and tissue deposition was studied in male Sprague-Dawley rats exposed to different levels of cadmium in the drinking water. In animals fed a high-protein or low-protein diet and drinking water containing 25 or 50 ppm cadmium, liver and kidney cadmium and metallothionein were both significantly higher in rats fed the high-protein diet for 2 to 4 months. These differences may possibly be explained by the concentration of cysteine observed between these two diets. When cysteine was added to the low-protein diet to the level observed in the high-protein diet and fed to rats receiving 25 ppm cadmium in the drinking water, significant dietary differences in liver and kidney cadmium and metallothionein were not observed. The importance of dietary protein to cadmium-induced toxicity was also assessed in these studies. The activity of catechol-o-methyltransferase was used as a measure of cadmium-induced toxicity. The activity of this enzyme in the lung, liver and heart was significantly lower in rats fed a low-protein diet than those fed the high-protein diet and 50 ppm cadmium. Metallothionein concentration in the lung and liver from low-protein-fed rats was approximately half the level observed in rats fed the high-protein diet, which suggests a relationship between cadmium-induced toxicity and metallothionein concentrations. These results illustrate the importance of considering dietary protein (and possibly cysteine) when studying cadmium metabolism in experimental animals.