The in vitro uptake of cycloleucine, histidine, valine and [alpha]-methyl-D-gluco-pyranoside into intestinal segments of hypothyroid rats has been studied. Pieces of jejunum were obtained from animals that had been rendered hypothyroid by thyroidectomy followed by administration of 0.5 mc I131, as well as from control euthyroid animals. With each substance, and at all time points studied, there was an increased intracellular accumulation in the hypothyroid group. This effect was diminished by treating the animals with parenteral triiodothyronine 15 [mu]g for 4-5 days. When valine accumulation was investigated in more detail, it was demonstrated that there was no difference in the efflux of amino acid from the segments in the hypothyroid and control animals. Michaelis-Menten studies showed that intestine from hypothyroid animals had a higher velocity of transport than either the control or treated hypothyroid group. Thyroxine added to incubation flasks containing tissue from the hypothyroid rats did not alter the accumulation of valine. It is postulated that in hypothyroidism there may be an alteration of the transport carrier to give an increased rate of amino acid and sugar transport into the intestinal mucosal cell.