Simultaneous infusion of glucose and Intralipid was carried out on dogs to evaluate the effect of glucose on utilization of the fat emulsion. For control experiments, normal saline was used in place of glucose. Results of these studies revealed that clearance of Intralipid and also of triglyceride and free fatty acid (FFA) from the plasma did not differ between the dogs receiving glucose and those receiving saline infusion. The rates of Intralipid clearance were 0.049 +/- 0.003 and 0.047 +/- 0.003 per minute (n = 16), respectively. Study with radioactive [14C]Intralipid also gave similar rates of 14C clearance from the plasma, 0.037 +/- 0.003 and 0.040 +/- 0.003 per minute (n = 4), respectively for the two groups. These suggested that clearance of Intralipid from the plasma was not affected by glucose. However, data on expired 14CO2 revealed that oxidation of the fat, which yielded CO2 as end product, was inhibited by glucose. The amount of 14CO2 expired in 2 hours was 10.4 +/- 1.4% for the saline group, which was significantly reduced to 3.0 +/- 0.4% (n = 4; p less than 0.01) for the glucose group. In each group, CO2 production was much delayed as compared to fat clearance from the plasma. Thus, the rate of clearance of Intralipid from plasma does not reflect the actual utilization of fat emulsion. In accordance with an experiment with insulin, it appears that suppression of CO2 production is directly related to plasma insulin levels.