1. The rate of metabolism of glucose to lactate has been measured in a number of non-vascularly perfused preparations of rat jejunum in vitro. The glucose and lactate metabolism was measured simultaneously and under conditions such that the uptake of glucose and the appearance of lactate were linearly related to time. 2. It is found that there is no difference between the rates at which rings of rat jejunum utilize glucose during the first 45 min of anaerobic or aerobic incubation. During the first 15 min of incubation between 60-70% of the metabolized glucose was converted to lactate under aerobic conditions; this value increased to 80-90% during the subsequent 30 min. During the period 0-15 min of incubation, lactate production was found to be higher under anaerobic than under aerobic conditions but after this initial period the rate of lactate production was the same under aerobic and anaerobic conditions. 3. For segments of rat jejunum, maintained in vitro by the recirculation of nutrient fluid through the intestinal lumen, neither the rate of production of lactate, nor the utilization of glucose, was stimulated if the preparation was maintained under anaerobic rather than aerobic conditions. The direct delivery of gas into the intestinal lumen in the form of a stream of bubbles (segmented circulation) reduced both the utilization of glucose and the production of lactate under aerobic conditions. However, not effect on glucose metabolism was observed under anaerobic conditions. The finding of a Pasteur effect with the segmented-circulated preparation, but not with the simple recirculated preparation, is associated with lower rate of aerobic lactate production in the former preparation. Reasons are given for supposing that under conditions of segmented circulation, the luminal compartment is better stirred, thereby increasing access of O2 to the tissue. 4. A preparation of rat small intestine perfused through the vascular bed is described. With this preparation the rate of glucose utilization is significantly lower than that for recirculated preparations and the rate of lactate production is substantially less than that of the other preparations studied. 5. With the preparation perfused through the vascular bed, and with glucose, 10 mM, present only in the vascular medium the addition of erythrocytes to the vascular infusate causes a significant reduction in both glucose utilization and in the rate of lactate production. The addition of erythrocytes to produce an haematocrit of 40% (v/v) causes a greater reduction in glucose utilization and lactate production than is found for an haematrocrit of 15%. About 10% of the lactate produced appears in the luminal contents. With an haematocrit of 15%, the O2 consumption of the whole wall of the jejunum was found to be 6-4 mumole O2 g dry wt.-1 min-1, equivalent of a value for the Q02 of 8-6 mul. O2 mg druwy wt.-1 hr-1. The uptake of O2 was almost entirely from the vascular infusate. 6...