The aim of this study was to evaluate endothelin-1 and prostacyclin production by human endothelial cells cultured in the presence of polyethylene terephthalate and collagen-coated PET. Cell counting and the assay of endothelin-1 and 6-keto-prostaglandin F1 alpha, stable metabolite of prostacyclin, were carried out after 48 hour contact of the cells with the examined materials. Endothelial cell contact with uncoated PET caused a significant reduction in cell number, a significant increase in the production of endothelin-1 and a not significant increase in 6-keto-prostaglandin F1 alpha. The endothelial cell contact with collagen-coated PET caused a highly significant decrease in cell number and a not significant decrease in endothelin-1 and 6-keto-prostaglandin F1 alpha. It was concluded that PET causes both a decrease in cell number and a remarkable increase in endothelin-1. On the contrary, collagen-coated PET determines a decrease in cell number and a slight reduction of endothelin-1 and 6-keto-prostaglandin F1 alpha.