Experimental studies have shown that ketone body infusion inhibits the catecholamine response to hypoglycemia. We have studied six men during insulin-induced hypoglycemia who were concomitantly infused with saline or 3-hydroxybutyrate at dose rates of 4 and 13 mg/kg.min. The mean blood concentrations of 3-hydroxybutyrate increased to about 0.45 and 3 mmol/liter, respectively. The results show that the glucose response to insulin; hypoglycemic symptoms; and the adrenaline, noradrenaline, and cortisol responses to hypoglycemia were not influenced by ketone infusion at any dose rate.