Our fetal surgical model was utilized to perform in utero fetal lamb thyroidectomy and autograft transplantation of thyroid tissue to fetal thigh at 82-93 days gestation. Successful in utero transplantation was possible in two of six experimental animals. In one twin pregnancy with an unoperated control lamb, observations were continued to age six months. The athyrotic lamb with a thigh autograft was larger at birth and had a transient weak sucking reflex and awkward gait. It then grew and developed normally with no stigmata of cretinism or delay in bone maturation. At age six months an increase in thyroid stimulating hormone (oTSH) was the single distinguishing observation in the twin with the transplant. Although oTSH levels were elevated to age six months, the pituitary continued to be responsive to thyrotropin releasing hormone (TRH) stimulation. These findings suggest that in utero transplantation of thyroid tissue is technically feasible and that the previously described development of in utero cretinism following fetal thyroidectomy can be prevented by a functioning autograft. This technique will be useful in attempting allograft transplantation in utero.