In response to homocysteine induced toxicity in human umbilical vein endothelial cells, minimal changes in the concentration of cellular protein thiols but substantial changes in the concentration of intracellular soluble thiols were observed. The latter correlated closely with changes in cellular glutathione levels. No correlation existed between cellular glutathione levels and cell viability, whereas a close correlation between NAD+ levels and cell viability was demonstrated. Large decreases in cellular NAD+ occurred in response to homocysteine induced toxicity which were accompanied by the production of single stranded DNA. 3-Aminobenzamide, an inhibitor of poly (ADP-ribose) polymerase preserved cell viability and cellular NAD+ levels. Evidence that DNA synthesis was also compromised was revealed by the decreased capacity of homocysteine treated cells to incorporate deoxyuridine. Radical scavengers were also effective in preventing homocysteine induced toxicity. It is likely that the major threat to cells derives from radicals generated intracellularly. Eicosanoid metabolism and the xanthine oxidase system have been identified as two potential sources of radicals.