Nicotinic acid 4 g daily was given to 28 weight-stable hypertriglyceridaemic patients. The aim was to study its effects on serum lipoprotein lipid levels, intravenous glucose tolerance (k-values) and glucose incorporation into subcutaneous adipose tissue (GLIAT) in vitro. The investigations were performed prior to the nicotinic acid therapy, after 6 weeks, and 6 months of drug treatment. Fasting blood glucose levels increased by 13%, whereas k-values fell by 26% after the nicotinic acid treatment. Decrease in k-values was predicted from the initial k-values (R2-value = 0.76). GLIAT increased by 76%, while in the subgroup of eight patients, treated for 6 months with nicotinic acid, GLIAT increased by 331%. The changes in k-values and GLIAT were not significantly interrelated. Serum triglyceride levels were strongly decreased. The most likely explanation for the decrease in intravenous glucose tolerance is that nicotinic acid stimulates glucose output from the liver and that this effect outweighs the stimulating effects of the drug on glucose utilization in extrahepatic tissues. The latter is reflected by the increased uptake of glucose in adipose tissue. A stimulated GLIAT, reflecting formation of alpha-glycerophosphate in adipose tissue, might contribute to the reduction of serum triglyceride levels induced by nicotinic acid, since alpha-glycerophosphate is the acceptor of fatty acids assumed to be liberated from circulating triglycerides by lipoprotein lipase.