The present study was designed to test the hypothesis that homocysteine (Hcys) reduces intracellular nitric oxide (NO) concentrations ([NO]_i) and stimulates superoxide (O·) production in the renal arterial endothelium, thereby resulting in endothelial dysfunction. With the use of fluorescence microscopic imaging analysis, a calcium ionophore, A-23187 (2 µM), and bradykinin (2 µM) were found to increase endothelial [NO]_i in freshly dissected lumen-opened small renal arteries loaded with 4,5-diaminofluorescein diacetate (DAF-2DA; 10 µM). Preincubation of the arteries with L-Hcys (20-40 µM) significantly attenuated the increase in endothelial [NO]_i. However, L-Hcys had no effect on NO synthase activity in the renal arteries, as measured by the conversion rate of [³H]arginine to [³H]citrulline, but it concentration dependently decreased DAF-2DA-sensitive fluorescence induced by PAPA-NONOate in the solution, suggesting that L-Hcys reduces endothelial [NO]_i by its scavenging action. Because other thiol compounds such as L-cysteine and glutathione were also found to reduce [NO]_i, it seems that decreased NO is not the only mechanism resulting in endothelial dysfunction or arteriosclerosis in hyperhomocysteinemia (hHcys). By analysis of intracellular O· levels using dihydroethidium trapping, we found that only L-Hcys among the thiol compounds studied markedly increased O· levels in the renal endothelium. These results indicate that L-Hcys inhibits the agonist-induced NO increase but stimulates O· production within endothelial cells. These effects of L-Hcys on [NO]_i and [O·] may contribute to endothelial injury associated with hHcys.