The insulin response of 10 lean and 23 obese subjects with lag-type and borderline O.G.T.T. has been studied. The O.G.T.T. was interpreted according to the criteria of Fajans and Conn. The maximum increase and the area of increase were examined both for blood glucose and plasma I.R.I., and the corresponding I.R.I./glucose ratios calculated. The shape of the insulin response curve is similar to that of glucose curve. The I.R.I./glucose ratios are decreased in the lag-type curves as compared to borderline in the lean subjects while we observed opposite results in obese ones. A possible physiopathological interpretation of this curves is proposed.