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Cultured human umbilical vein endothelial cells treated with either the calcium ionophore, ionomycin, or ionomycin plus cyanide-m-chlorophenylhydrazone had immediate severe depletion of adenosine triphosphate, (ATP) and increases of cytosolic free calcium (Caf) and then sustained lethal cell injury as manifested by release of lactate dehydrogenase and failure to exclude vital dyes within 15 minutes. Inclusion of glycine in the experimental medium prevented the enzyme leakage for at least 60 minutes without altering the ATP depletion or increases of Caf. The physiologic glycine concentration of 0.25 mmol/l gave 50% protection, and protection was complete at 1 mmol/l. Several other small neutral amino acids, L- and D-alanine, beta-alanine, 1-aminocyclopropane-1-carboxylate, alpha-aminoisobutyrate, and L-serine, had effects similar to glycine, but other amino acids and metabolic substrates did not. The endothelial cells were relatively resistant to damage from hydrogen peroxide, but sensitivity could be increased by preloading with Fe2+. In both non-loaded and Fe(2+)-loaded cells, hydrogen-peroxide-induced lactate dehydrogenase (LDH) release developing over 180 minutes was prevented by glycine in a fashion analogous to that seen with ionomycin damage. Mn2+ also partially protected against hydrogen peroxide injury but was not required for glycine's effects. These data demonstrate that striking modulatory effects of glycine and structurally similar amino acids that have previously been characterized in most detail using kidney tubule cells are strongly expressed in human umbilical vein endothelial cells and are involved in their response to Ca2+ and oxidant-mediated damage. These amino acid effects must be considered in the design of in vitro studies of endothelial cell injury and may contribute to endothelial cell pathophysiology in vivo.