Background/Aims: Experimental studies suggest an enhanced endothelial and platelet nitric oxide (NO) generation after statin treatment, possibly due to increased endothelial NO synthase (eNOS) activity and protein levels. In parallel with experimental research, statins were shown to increase the forearm blood flow independently of serum cholesterol in humans. However, it was not possible to correlate blood flow changes with eNOS levels in these studies due to limitations in obtaining arterial samples. Hence, we investigated changes in eNOS activity, mRNA and protein levels after statin treatment in human platelets, which are readily accessible unlike arteries. Methods: In vitro bleeding times were measured in 22 patients by stimulating platelets with collagen-epinephrine or collagen-ADP. To assess platelet eNOS activity, the bleeding times were also determined after incubating platelets with L-arginine. The measurements were repeated following 14 days of pravastatin (40 mg/day) treatment. Platelet-rich plasma was collected before and after statin treatment to evaluate eNOS mRNA (semiquantitative RT-PCR) and protein levels (Western blotting). Results: The basal bleeding time was prolonged by 24 +/- 3% (mean +/- SE) when the samples were incubated with 500 mu M of L-arginine. The NOS inhibitor L-N-5-(I-iminoethyl) ornithine reversed this effect, suggesting that it was mediated by NO. After statin treatment, the NO-mediated prolongation of the bleeding time with 500 mu M of L-arginine was significantly potentiated (to 44 +/- 10%). Despite enhanced eNOS activity, there was no significant change in platelet eNOS mRNA and protein levels after statin treatment. Conclusion: These data demonstrate that platelet eNOS activity is potentiated after statin treatment in humans in parallel with experimental studies. Copyright (C) 2008 S. Karger AG, Basel.