The metabolism of vitamin D3-3H was studied in a small group of controls and subjects with tropical sprue after the oral or intravenous administration of 8 to 10 microCi of D3-3H. The biological half life of D3-3H upon the administration of the isotope by the intravenous route was normal in 2 controls, very low in a subject with tropical sprue who had steatorrhea, and decreased in a subject with tropical sprue who did not present steatorrhea. After the administration of the isotope by the oral route, the biological half life was 35 hours in the control and no radioactivity could be detected in the plasma of the subject with tropical sprue who had steatorrhea. Twenty four hours after the intravenous dose the percentage of radioactivity in the plasma as HCC-3H was two times higher in the tropical sprue subjects than in the controls. When the dose was given orally the net absorption was 50.5% in the subject with tropical sprue and steatorrhea and 86.8% in the subject with tropical sprue who was partially treated. These results showed rapid clearance of the D3-3H in the subject with tropical sprue and steatorrhea indicating depletion of vitamin D stores in the tissues and decrease in the net absorption of the dose when given orally. The presence of a higher percentage of the dose in the plasma as HCC-3H after the intravenous and oral administrations in the tropical sprue subjects when compared to controls indicates that the diseased state does not alter vitamin D3 metabolism.