The ability of low phosphorus diets to stimulate the activity of the 25-hydroxyvitamin D3-1alpha-hydroxylase was tested in the chick. Feeding low phosphorus diets for 2 weeks resulted in a marked increase in enzyme activity relative to chicks fed a normal phosphorus diet. Stimulation of the 25-hydroxyvitamin D3-1alpha-hydroxylase activity by low phosphorus diets, however, was not as great as that observed with a low calcium diet. The low phosphorus and low calcium diets probably results from increased 1,25-dihydroxyvitamin D3 synthesis, whereas the stimulation by phosphate deprivation is only partly the result of increased 1,25-dihydroxyvitamin D3 production.