This study examined the effects of high and low glycemic index (GI) carbohydrate preexercise meals (2.5 g CHO/kg body mass) on cycle performance in untrained females. Ten females, cycled at 60% VO_2max to exhaustion, on two occasions. After fasting, subjects ate an isocaloric, high glycemic index (HGI)/low glycemic index (LGI) meal in a random order. Blood samples were taken at rest/postprandial/during and after exercise and blood glucose and lactate were measured. Ingestion of the LGI meal resulted in a performance time of 67.4 ± 8.4 min versus an HGI time of 48.9 ± 10.0 min (p = 0.02). Fifteen minutes after the HGI meal there was a significant increase (p < 0.001) in glucose levels, which was not seen in the LGI trial. Twenty minutes into the HGI exercise trial, there was a large decline in blood glucose concentration beyond resting levels. Based on this work, we found that untrained female participants should utilize LGI meals preexercise for endurance activities rather than HGI meals.