Circulating concentrations of 25-hydroxyvitamin D (25-OHD) were measured during short-term and long-term oral treatment with 25-hydroxycholecalciferol (25-OHD3), 25-H.C.C.) or with vitamin D in over 200 subjects over a period of 5 years. Ten times more vitamin D than 25-OHD3 was required to produce equivalent plasma-25-OHD concentrations. Plasma-25-OHD was a power function of dosage with both compounds. These data indirectly measure the superior therapeutic potency of 25-OHD3, show that dose-response relations with both compounds may be useful in diagnosis, and indicate that there are pronounced constraints on 25-hydroxylation of vitamin D. Together with the effects of ultraviolet light, now shown to be equivalent to oral vitamin D in doses of 8000-10 000 I.U. daily, these constraints may protect against vitamin-D deficiency in winter.