['Oral submucous fibrosis is a pre-malignant fibrotic condition caused by areca nut use and involves reduced mucosal vascularity. Arecoline is the principal areca nut alkaloid and is cytotoxic for epithelium and fibroblasts. Endothelial cell cycle arrest is reported on exposure to arecoline, as is cytotoxicity for endothelial-lung carcinoma hybrid cells. We here describe cytotoxicity for primary human endothelial cultures from seven separate donors.', 'Human umbilical vein endothelial cells were exposed to increasing concentrations of arecoline and examined by: phase-contrast microscopy, haemocytometer counts, transmission electron microscopy, lactate dehydrogenase release and the methyl-thiazol-tetrazolium assay.', 'Vacuolation and detachment of endothelium were observed at and above arecoline concentrations of 333\xa0μg/ml or more. Ultrastructural features of cellular stress were seen after 24-h treatment with 111\xa0μg/ml arecoline and included reduced ribosomal studding of endoplasmic reticulum, increased autophagolysosomal structures, increased vacuolation and reduced mitochondrial cristae with slight swelling. Similar changes were seen at 4\xa0h with arecoline at 333\xa0μg/ml or above, but with more severe mitochondrial changes including increased electron density of mitochondrial matrix and greater cristal swelling, while by 24\xa0h, these cells were frankly necrotic. Haemocytometer counts were paralleled by both lactate dehydrogenase release and the methyl-thiazol-tetrazolium assays.'] Is arecoline cytotoxic for human endothelial cells?