Cerebral function was measured with a neuropsychological test battery before, during, and after insulin-induced hypoglycaemia (blood glucose approximately 2.0 mmol l-1) in 10 male Type 1 diabetic patients (age 20-43 years, duration of diabetes 14 (2-30) years) and in 12 normal men. There were no group differences in neuropsychological results at normal glucose levels. Significant effects of hypoglycaemia were found in reaction-time measures (p less than 0.001) and in other tests requiring speed and attention (p less than 0.001), in verbal fluency (p less than 0.05), and short-term memory (p less than 0.001). Significant group effects and interactions (p less than 0.05) revealed that the diabetic patients were generally more affected by hypoglycaemia than the normal subjects. This might have been partly due to the larger absolute decrease in blood glucose level in the diabetic patients, although the rate of glucose decrease was not related to performance in either group. Thus, the diabetic brain might be more vulnerable to hypoglycaemia, perhaps through the persistent impact of repeated hypoglycaemic episodes, although no neuropsychological deficit is demonstrable at normal blood glucose levels.