Adult female Wistar rats receiving normal or vitamin D restricted diet were made thyrotoxic by subcutaneous injection of T3. T3 treatment enhanced intestinal alkaline phosphatase activity and augmented the stimulatory effect of vitamin D on the enzyme in this tissue. In kidney, T3 treatment lead to a decrease in alkaline phosphatase activity in normal, vitamin D restricted and vitamin D repleted rats. The conversion of 25OHD3 to 24,25(OH)2D3 in vivo was enhanced in thyrotoxic rats. These results suggest an interaction between the thyroid and vitamin D endocrine systems.