Abstract Thyroparathyroidectomised (T.P.T.X.) rats on a high-calcium diet were given a low-calcium, low-phosphate diet. There was a pronounced increase in production of 1,25-dihydroxycholecalciferol (1,25-D.H.C.C.), the active metabolite of vitamin D, which was not explained by a fall in plasma-phosphate. Thus, parathyroid hormone is not necessary for the initiation of increased production of 1,25-D.H.C.C. by the kidney, and is therefore not the trophic hormone for this renal endocrine system. A likely determinant of the type of vitamin-D metabolite produced is the intracellular calcium concentration within some critical compartment of the renal cell.