Insulin response and FFA behavior have been evaluated during an IVGTT in 63 subjects of whom 18 were normal, 31 were obese (with varying degrees of carbohydrate tolerance) and 14 were mild non insulin-dependent diabetics. The extreme reduction of insulin secretion in the early phase (Δ 0–15 min) and the less severe impairment of the late phase (Δ 15–60 min) have been confirmed; obese subjects showed on the average an active insulin response to venous loading; this was more marked and more consistent in the late phase. Compared to controls, FFA concentration both in basal conditions and during IVGTT was progressively higher in obese and diabetic patients. When analyzing the interplay between IRI, K_G and FFA in the course of IVGTT, it was observed that: (1) a close correlation exists between K_G and early insulin response (r=0.72); (2) a correlation between Δ IRI 0–15 min and percentage decrease of FFA at 45 min is found only in normal subjects; (3) a negative highly significant correlation is found between K_g and mean FFA plasma level 0–60 min. This last correlation is evidence of the important role played by FFA in carbohydrate tolerance. The conflicting results reported by others have been discussed.