The aim of this study was to investigate the effects of acute pharmacological treatment on the plasma levels of l-arginine, asymmetrical dimethylarginine (ADMA), and symmetrical dimethylarginine (SDMA). We also investigated the related effects on endothelial nitric oxide synthase (eNOS) expression and activity and cytochrome c oxidase activity in the primary blood mononuclear cells (PBMCs) isolated from patients with acute congestive heart failure (ACHF). Compared to pre-treatment values, ADMA, SDMA, and l-arginine plasma levels were significantly higher after pharmacological treatment (ADMA, 0.82 versus 0.43 µM; SDMA, 1.52 versus 1.12 µM; l-arginine, 1.78 versus 1.29 µM; p < 0.01. In addition, the levels of eNOS expression and activity were decreased after pharmacological treatment, while cytochrome c oxidase activity resulted in higher O2-production. In the PBMCs isolated from patients with acute congestive heart failure (ACHF) and impaired renal function, higher SDMA and ADMA levels were more evident after therapy, as were reduced expression and activity of eNOS. Increased O2- produced after treatment may be involved in impaired recovery of cardiac function associated with higher plasma levels of SDMA.