In order to decide whether the phospholipid fatty acid pattern is related to variables determining glucose tolerance, 11 healthy volunteers with normal glucose tolerance were studied. The relationship was evaluated between the proportions of individual fatty acids (FA) in serum phospholipids and (1) insulin secretion, determined by fasting and postglucose plasma insulin levels, and (2) in vivo insulin action, assessed as metabolic clearance rates of glucose during euglycemic clamp studies at two insulin concentrations of approximately 70 microU/mL (MCRglu70) and 500 microU/mL (MCRglu500). It was found that both insulin secretion and insulin action are significantly related to the ratio of omega-6 class essential FA to saturated FA in serum phospholipids. An increase of this ratio is associated with a decrease in total insulin response (r = -0.84, P less than .01), and an increase in MCRglu70 (r = .66, P less than .05) and MCRglu500 (r = .82, P less than .01). The data presented support the hypothesis that phospholipid FA composition might play a role in blood glucose regulation.