The effect of furosemide on carbohydrate metabolism was studied in mice. Single‐dose administration (200 mg kg‐1body weight) resulted in transient hyperglycaemia and a rise in the glucose/insulin ratio within 60 min. The glucose tolerance was impaired with elevated serum glucose and reduced insulin response 2 h after the furosemide injection, but had recovered within 24 h. In mice made hypoglycaemic by prior injection of insulin, the basal serum glucose and the glucose tolerance were impaired 22 h after the injection of furosemide. It is suggested that furosemide has both acute and long‐term effects on carbohydrate metabolism in mice and that, at least in part, this is due to reduced insulin secretion. Glucose may protect against the diabetogenic action of furos