The relationship between performance on a dichotic listening task and blood glucose levels was examined. It was predicted that, if the ability of blood glucose to supply the brain with its basic fuel limits performance under conditions of cognitive demand, in a dichotic listening task information directed to the left hemisphere would be particularly susceptible to the level of blood glucose. Via headphones subjects heard lists of words directed to both ears, although randomly they were asked to attend to those directed to one ear. Those receiving a glucose drink recalled more of a list of words directed to the right ear and hence the left hemisphere. Subjects with low baseline blood glucose levels recalled more from the attended ear, and those with high baseline blood glucose more from the unattended ear. In those who received a glucose drink, a fail in blood glucose during the dichotic task was associated with a right ear superiority. The pattern of findings can be explained by the assumptions that under conditions of cognitive demand the supply of glucose to the brain limits performance and that there are individual differences in the ability to efficiently take glucose from the blood stream into the brain.