The effect of experimental hypothyroidism on the absorption, transmural transport and metabolism of glucose was studied by perfusion of isolated loops of rat jejunum in vitro. When expressed on a dry weight basis, the rate of absorption was enhanced by 32% (P < 0.01); when expressed on a length basis there was no significant change, since the enhancement per unit weight was almost exactly compensated by a diminution in mass per unit length in the hypothyroid state. When expressed in either units, there was a significant enhancement in transmural transport (+123% and +77%, respectively, both P < 0.001), which reflected in part a diminution in the rate of glucose utilization (-29%, P < 0.01 and -43%, P < 0.001, respectively). The changes in glucose utilization were matched by changes in lactate production. Three factors contributed to the diminution in glucose utilization in the hypothyroid state: a diminution in the concentration of 6-phosphofructo-1-kinase (-35%, P < 0.05), and increase in the S0.5 of 6-phosphofructo-1-kinase for fructose 6-phosphate from 0.4 to 0.6 mM and a fall in the mucosal concentration of fructose 2,6-bisphosphate (-56%, P < 0.05). From the point of view of the whole animal, there is little if any change in the capacity of the intestine to absorb glucose from the lumen, but there is a large enhancement of transmural transport that is metabolically driven.