Reduced prostacyclin (PGI2) production by the vascular wall has been proposed as one of the possible causes of diabetic vascular complications. We found an activity which stimulated PGI2 production by cultured endothelial cells (PGI2-stimulatory activity, PSA) in human plasma-derived serum (PDS). The PSA was less in patients with diabetes mellitus. The present study was undertaken to evaluate how metabolic factors relevant to diabetic angiopathy modify the PSA. Pooled PDS was prepared from 10 healthy volunteers. The 6-keto-PGF1 alpha (6KF, a stable metabolite of PGI2) production by cultured bovine aortic endothelial cells was maximally stimulated by Dulbecco's modified Eagle's medium (DMEM) containing 10% pooled PDS after incubation for 60 min. The production of 6KF was reduced in a dose-dependent manner by the addition of 10% pooled PDS with glucose and linoleic acid hydroperoxide (lipid peroxide). In contrast, human low density lipoprotein (LDL) and linoleic acid (unsaturated fatty acid) enhanced the production of 6KF by 10% pooled PDS in a dose-dependent manner. Insulin, however, showed no effect on the production of 6KF by 10% pooled PDS. These results suggest that the reduced PSA in diabetics may be the result, in part, of a modification of the PSA by diabetic metabolic factors such as glucose and lipid peroxide.