Recent studies have suggested that part of the vasorelaxation caused by nifedipine, a 1,4-dihydropyridine Ca2+antagonist, depends on the endothelium. To study the effect of endothelium-dependent vasorelaxation, the release of NO and superoxide (O2−) in the presence of nifedipine in isolated cultured rabbit endothelial cells was measured. Highly sensitive electrochemical microsensors were placed onto the cell membrane, and the kinetics of NO and O2−were measured simultaneously with time resolutions of 0.1 and 0.05 ms, respectively. Nifedipine at its therapeutical concentrations stimulated NO release and scavenged O2−in endothelial cells. The linear relationship between NO concentration and nifedipine concentration was observed in the range between 0.01 and 1 nmol/L. NO concentration reached a maximum of 200±10 nmol/L at 1.2 nmol/L of nifedipine. The NO concentration was ≈50% and 30% of the concentration measured in the presence of receptor-dependent (acetylcholine) and the receptor-independent (Ca2+ionophore A23187) NO synthase (eNOS) agonists, respectively. NO release stimulated by eNOS agonists was followed by the generation of the NO scavenger superoxide. The concentration of O2−was significantly lower after stimulation with nifedipine (peak 5±0.5 nmol/L) than after stimulation with acetylcholine (15±1 nmol/L) and Ca2+ionophore (25±1 nmol/L). The average rate of NO release by nifedipine is relatively slow (17 nmol/L per second). This is in sharp contrast to the fast rate of NO release by acetylcholine and Ca2+ionophore (40 and 300 nmol/L per second, respectively). These experiments show that nifedipine, apart from its well-known Ca2+antagonistic properties in vascular smooth muscle cells, stimulates the release of significant concentration of NO in endothelium and also preserves NO concentration. Both these effects may be beneficial in the treatment of hypertension.