This study examined why decreased protein intake retards zinc deficiency in zinc-deficient rats. Rats were freely provided zinc-deficient diets with either 10 or 20% protein. Experimental groups consisted of five rats that were fed experimental diets for 0, 3, 4, 12 and 25 d in Experiment 1. The body protein content in rats fed the 10% protein diet was similar to those fed the 20% protein diet for the duration of the experiment. The body zinc content in both dietary groups slowly decreased in a similar manner. Eventually, the body zinc/protein ratio in the 10% protein diet group decreased more slowly than that in the 20% protein diet group. Ingestion of the 10% protein diet also reduced the zinc/protein ratio in bone more slowly compared with that of the 20% protein diet, under zinc-deficient conditions, at 12 d in Experiment 2. However, there was no difference in the zinc/protein ratio of carcass total soft tissue between the two zinc-deficient groups. Decreased protein intake eventually slowed the reduction in both the body and bone zinc/protein ratios in zinc-deficient rats, resulting in retardation of zinc deficiency.