Previously it has been found that both missing breakfast and having poorer glucose tolerance were associated with better memory. The present study therefore examined the impact of eight breakfasts, in a factorial design, that contained either high or low levels of carbohydrate, fat or protein. The meals were designed to vary the rate of release of glucose into the blood stream. Memory, reaction times and vigilance were assessed 30, 75 and 120 min following breakfast. Using fasting blood glucose levels as a measure of glucose tolerance, better memory was found to be associated with better glucose tolerance and the consumption of meals that more slowly release glucose into the blood. The effects of the meals on reaction times and vigilance were opposite to those with memory in that higher levels of blood glucose tended to be associated with better performance. It was concluded that individual differences in glucose tolerance interact with the glycaemic load of a meal to influence cognitive functioning.