Prostacyclin (PGI₂) is a potent vasoactive prostanoid regulating vascular tone. We recently purified and cloned a PGI₂-stimulating factor (PSF), which stimulates PGI₂ production by vascular endothelial cells (ECs). Previous study demonstrated that PSF is predominantly located in vascular smooth muscle cells (SMCs) and present in serum. PSF may act on vascular ECs to regulate PGI₂ synthesis for maintaining vessel wall homeostasis. Decreased PSF production in the vessel wall may result in an imbalance of prostanoid synthesis, leading to the development of vascular lesions such as diabetic angiopathy. In the present study, to investigate the regulatory mechanisms of PSF gene expression, we examined the effect of high glucose concentrations on PSF mRNA expression in cultured bovine aortic SMCs. Expression of PSF mRNA was significantly decreased to 66 ± 6 % of control value (p < 0.01), when the glucose level was raised from 5.5 to 27.8 mmol/l. We also examined the effect of osmolarity on PSF mRNA expression by addition of an appropriate dose of mannitol to the culture medium. We confirmed that high glucose concentration itself reduced the expression of PSF mRNA and glucose had much more effect than the osmolarity control. The expression of PSF mRNA was significantly decreased to 72 ± 5 % of control value (p < 0.05) by a protein kinase C (PKC) activator, phorbol-12-myristate-13-acetate (PMA). The decreased expression of PSF mRNA in the presence of high glucose or PMA was restored by co-incubation with a PKC-specific inhibitor (GF109203X). These results suggest that PSF gene expression in vascular SMCs may be decreased via a specific effect of high glucose concentrations. High glucose-induced activation of PKC is suggested to participate partly in the regulation of PSF gene expression. [Diabetologia (1998) 41: 134–140]