Cognitive functioning was assessed in diabetic patients during hypoglycemia (60 mg/dl), euglycemia/ control (110 mg/dl), and hyperglycemia (300 mg/dl). Blood glucose levels were set and maintained to within 4% of targeted levels by an artificial insulin/glucose infusion system (Biostator). Attention and fine motor skills, assessedby visual reaction time, was slowed at altered glucose levels. Performance was less impaired during hyperglycemia than hypoglycemia when a longer interstimulus interval was used,although it was still slower than normal. The time required to solve simple addition problems was increased during hypoglycemia, although reading comprehension was not affected. The possibility that some automatic brain skills are disrupted at altered glucose concentrations is discussed, while associative or inferential skills may be less affected.