Background: Nitric oxide (NO) is a crucial signaling molecule which regulates the blood flow and prevents the adhesion of blood components to the vascular wall. A deficiency in bioavailable NO concentration is associated with the dysfunction of endothelial NO synthase (eNOS) and/or an increase in oxidative stress. The deficiency of bioavailable NO is a common denominator of several cardiovascular diseases, including diabetes, atherosclerosis, and hypertension. Materials and Methods: We used a nanomedical technology to elucidate the balance between bioavailable NO and oxidative stress (peroxynitrite ONOO−) in human umbilical vein endothelial cells (HUVECs) treated with a supplement containing L‑arginine, L‑citrulline, Vitamin D3, and antioxidants. Nanosensors, with a diameter of 200–300 nm, are capable of measuring in situ NO and peroxynitrite (ONOO−) concentrations produced by single endothelial cells. Results: The ratio of the concentration of cytoprotective NO [NO] to the concentration of cytotoxic peroxynitrite [ONOO−] was used to estimate the efficiency of eNOS. HUVECs incubated with L‑citrulline, L‑arginine, and Vitamin D3 increased the [NO]/[ONOO−] ratio by 25%, while in the presence of antioxidants, the increase was 15%. The synergistic effect between the mix of L‑arginine, L‑citrulline, Vitamin D3, and antioxidants was a favorable increase of the overall [NO]/[ONOO−] ratio by 50%. Conclusion: The findings of the study presented here clearly indicate that L‑arginine, L‑citrulline, and Vitamin D3 can significantly alter the function of the endothelium and NO production, in a favorable manner, while pointedly reducing ONOO− – the main component of oxidative stress. This effect can be significantly potentiated in the presence of antioxidants.