25 OH D3 plasma levels were measured in thyrotoxic patients and in euthyroid subjects before and after the intravenous injection of vitamin D3. In thyrotoxic subjects the transformation of injected vitamin D3 into 25 OH D3 had already begun six hours after the injection, reached a maximum between the twelfth and thirty-sixth hour and decreased thereafter. In contrast, in euthyroid healthy subjects the increase in 25 OH D3 plasma levels starts 96 hours after the vitamin D3 injection. On the basis of these data it may be estimated that hepatic hydroxylation of vitamin D3 in thyrotoxicosis is 2.5 faster than in euthyroid healthy subjects. This finding may explain the variability in 25 OH D3 plasma levels reported in thyrotoxicosis, the final value depending on degree of acceleration of the hepatic hydroxylation of vitamin D3 and on the vitamin D3 status of the organism.