Acute hypoglycemia provides a reproducible method of investigating the effect of biological changes induced during hypoglycemia on mood states. Hypoglycemia was induced twice using a hyperinsulinemic glucose clamp in 24 nondiabetic human participants; a euglycemic placebo control study was also performed. Serial changes in mood were assessed using the UWIST Mood Adjective Checklist before, during, and after 60 min of controlled hypoglycemia (2.5 mmol/l). Hypoglycemia induced a significant reduction in hedonic tone (p = .001), a significant increase in tense arousal (p < .0005), and a significant decline in energetic arousal (p = .01) in comparison with the euglycemia control study. Profound changes in mood were observed in nondiabetic participants during acute hypoglycemia, and a state called tense tiredness persisted for at least 30 min after restoration of euglycemia.