The nature of the suppression of the pituitary-thyroid axis during infection was studied by testing the integrity of thyrotropin (TSH) and prolactin (PRL) responses to thyrotropin-releasing hormone (TRH) during acute falciparum malaria in human volunteers. During infection, TSH responses to TRH were found to be intact while PRL secretion was slightly increased. That serum T3 levels abruptly declined during infection while serum T4 was stable or increasing suggested an alteration in peripheral degradative pathways and prompted the measuremnt of reverse T3. Changes in serum T3 concentration were found to be accompanied by reciprocal changes in reverse T3. These observations allow some clarification of previously unknown aspects of thyroidal economy during infection.