The glucose infusion test (GIT) was performed in 10 controls, 30 obese non-diabetic subjects and 10 obese insulin-nondependent diabetics. After a priming glucose injection of 500 mg/kg, glucose was infused during one hour at a rate of 20 mg/kg/min. In healthy persons there was a biphasic plasma insulin curve with the first peak 3 min. after the start of glucose injection and a second one at 60 min. In 20 obese subjects with normal OGTT, both the initial and the later plasma insulin responses to glucose infusion were higher than in controls. In 10 obese subjects with mild glucose intolerance the average initial plasma resembled that of obese subjects with a normal OGTT. In overt mild diabetes the insulin levels during the initial phase of the GIT were diminished, but a somewhat higher, though markedly delayed, rise of insulin levels could be shown during the second hour. When calculated as sums of plasma insulin increments at 3 + 5 min., one low response in 10 controls, no low response in 20 obese subjects with a normal OGTT, but 5 low responses in 10 obese subjects with a border-line OGTT and 9 low or very low responses in 10 obese diabetics were observed.