To assess the influence of glucose ingestion on resting and exercise electrocardiograms, 35 subjects were tested after ingestion of a glucose solution and a placebo on separate days. Graded submaximal exercise tests (GXTs) were performed to 90% predicted maximal heart rate. Computer-processed ECGs taken at rest showed a decrease in T-wave amplitude, more ST-segment depression, increased R-wave amplitude, and increased heart rate after glucose. Minnesota code classifications of these same ECGs were more abnormal after glucose. Computer-processed exercise ECGs showed a decrease in T-wave area after glucose ingestion during and after exercise. ST-segment area showed the most variability at one minute after exercise. Three subjects had normal exercise ECGs after placebo and abnormal ECGs after glucose ingestion (ST area >7.5μv/sec). Both resting and exercise ECGs should be made fasting to avoid this significant source of variability.