The effects on lipoprotein and glucose metabolism of addition of n-3 fatty acids were studied in 14 non-insulin-dependent diabetic patients who were given 10 g of MaxEPA (3 g n-3 fatty acids) or placebo (olive oil) per day in a randomized double-blind cross-over study during two consecutive 8-week periods. After MaxEPA treatment, there was a marked increase in long-chain polyunsaturated fatty acids of the n-3 series in the plasma lipid esters and in the platelet phospholipids, while the n-6 fatty acid content decreased. The very low density lipoprotein (VLDL) triglyceride concentrations decreased significantly (by 22%) on MaxEPA treatment. However, these changes were not significantly different from those observed during the placebo period. The blood glucose concentration tended to increase during MaxEPA treatment, and to decrease during the placebo period, the changes under the two regimes being significantly different (P less than 0.01). In addition, the rate constant for glucose disappearance (k value) for the intravenous insulin-tolerance test, which reflected the peripheral insulin sensitivity, tended to decrease during MaxEPA treatment and increase during administration of the placebo, there being a significant difference (P less than 0.03) between the changes during the two treatments. The reason for the observed changes in blood glucose concentration and peripheral insulin sensitivity is still unclear.