To determine whether vasodilator prostaglandins are involved in the peripheral hyperperfusion observed in patients with short-term insulin-dependent diabetes mellitus (IDDM), forearm and skin blood flow were studied before and after cyclooxygenase inhibition. Skin nutritive (CBV: capillary blood-cell velocity) and thermoregulatory (LDF: laser-Doppler fluxmetry), and forearm (muscle) blood flow (FBF) were measured before and after 500 mg acetylsalicylic acid (ASA) infused intravenously in 14 short-term IDDM patients and 22 healthy control subjects. In the IDDM patients, baseline LDF (median: 27 (19-35); interquartile range) vs. 17 (15-23) pu) and FBF (3.4 (2.5-4.1) vs. 2.6 (2.2-2.9) ml 100 ml-1 min-1) were increased, while CBV (0.70 (0.40-1.33) vs. 0.69 (0.41-0.96) mm s-1) was unchanged compared to healthy controls. ASA infusion had similar effects on baseline CBV, LDF, and FBF in patients and controls. In eight of the control subjects the role of prostaglandins in the regulation of basal peripheral blood flow was studied before and after ASA and placebo infusion. The changes in baseline CBV, LDF, and FBF were similar after ASA and placebo infusion in healthy controls. In conclusion, in short-term IDDM patients, increased skin thermoregulatory and forearm (muscle) blood flow are probably not related to vasodilator prostaglandins. Furthermore, prostaglandins are not likely to be involved in regulating basal peripheral blood flow in healthy man.