The capacity of prostacyclin production determined as plasma 6-keto-PGF1 alpha was investigated in 12 type 1 (insulin-dependent) diabetic patients with a median duration of diabetes of 14 years during ordinary metabolic control. Using high pressure liquid chromatography preceding radioimmunoassay, the plasma concentration of 6-keto-PGF1 alpha, the stable metabolite of prostacyclin, was determined at rest and after a standardised bicycle exercise test. The plasma 6-keto-PGF1 alpha in diabetic patients at rest did not differ from that of 25 healthy volunteers; 2.9 pg/ml (range less than 0.2-15.3) versus 1.7 pg/ml (range less than 0.2-16.6). During the exercise test plasma 6-keto-PGF1 alpha increased significantly in the diabetic patients as well as in the control group (p less than 0.05). The increment of 6-keto-PGF1 alpha in the diabetic patients was neither related to the metabolic regulation, duration of diabetes nor to changes in platelet volume, platelet number or the production of thromboxane B2 and prostaglandin E2. Our results do not support the hypothesis that Type 1 diabetic patients have a decreased capacity of prostanoid production, as suggested from in vitro studies.