The effect of 3-methylpyrazole-5-carboxylic acid (MPC) on carbohydrate and lipid metabolisms was studied in 18 patients with diabetes mellitus. In addition to diet 17 patients had basic treatment with sulfonylureas with or without biguanides, one patient was treated with insulin. In all patients carbohydrate metabolism was not well controlled, 14 patients had elevated triglycerides. Following a control period of 2 weeks the patients received increasing doses of MPC in addition to basic treatment (25825825 mg; 50,25,25 mg; 50, 50, 25 mg). Blood samples were taken in the fasting state before the first dose of MPC. Free fatty acids almost doubled under the influence of MPC. This was due to a rebound effect at night following suppression of lipolysis during the day. Blood glucose levels showed a tendency to fall, urinary glucose excretion, separately examined for day and night ,did not change consistently. Triglycerides fell markedly by 25%, but this reduction was not statistically significant. Cholesterol decreased by 5%. 40% of the patients showed an increase in urinary ketone bodies. Body weight did not change. Side effects due to MPC included flushing, gastrointestinal distress and cardiovascular complaints and were observed in 75% of the patients. Due to the high frequency of side effects it does not seem to be worthwhile to further investigate the therapeutic effect of MPC in a larger number of patients with different dosage regimens.