The short-term effect of the lipid lowering agent nicotinic acid on circulating concentrations of insulin, glucose, lactate, pyruvate, non-esterified fatty acids (NEFA), glycerol, total ketone bodies and triglycerides was examined in six insulin-dependent diabetic patients. On two occasions a week apart 24h metabolic profiles were performed. Three patients received nicotinic acid (800 mg/day) for 1 week prior to the first study and three patients between studies. Using this dose of nicotinic acid in patients with insulin-dependent diabetes no lipid lowering effect was demonstrated, nor did we observe an impairment of glycaemic control. During treatment with nicotinic acid circulating free insulin concentrations were higher and blood glucose concentrations were lower. Despite the higher insulin concentrations, circulating levels of NEFA, ketone bodies, and glycerol were all significantly elevated during treatment with nicotinic acid. These results suggest that any extrapolation of findings with regard to the use of nicotinic acid and its derivatives in non-insulin-dependent diabetes to insulin-dependent diabetes should be considered with caution.