The aim of this study is to investigate the effects of exogenous L-arginine and HDL on LDL and oxidized LDL (ox-LDL)-mediated platelet activation. Adenosine diphosphate (ADP)-activated platelets have been incubated with lipoproteins with or without L-arginine. P-selectin receptor numbers per platelet have been measured by flow cytometry. After incubation with only L-arginine (without lipoproteins), platelet nitric oxide (NO) levels and P-selectin receptor numbers significantly increased compared to the controls (P < .05). After incubation with LDL or ox-LDL, receptor numbers of P-selectin significantly increased (P < .001). However, P-selectin receptor numbers in platelets treated with L-arginine + LDL or L-arginine + ox-LDL decreased compared to the levels in platelets treated with only LDL or ox-LDL (P < .01, P < .001, respectively). Addition of HDL to L-arginine + ox-LDL caused significant reduction in P-selectin receptor numbers as in the control values (P < .001). We have concluded that L-arginine causes enhanced platelet NO levels and blocks the effects of LDL or ox-LDL on platelet P-selectin receptor numbers and HDL also strengthens this effect of L-arginine.